[ruby/json] json_string_unescape: assume the string doesn't need escaping

If that assumption holds true, then we don't need to copy the
string into a buffer to unescape it. For small string is just saves
copying, but for large ones it also saves a malloc/free combo.

Before:

```
== Parsing twitter.json (567916 bytes)
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json    52.000 i/100ms
                  oj    61.000 i/100ms
           oj strict    70.000 i/100ms
          Oj::Parser    71.000 i/100ms
           rapidjson    55.000 i/100ms
Calculating -------------------------------------
                json    510.111 (± 2.9%) i/s    (1.96 ms/i) -      2.548k in   5.000029s
                  oj    610.232 (± 3.1%) i/s    (1.64 ms/i) -      3.050k in   5.003725s
           oj strict    713.231 (± 3.2%) i/s    (1.40 ms/i) -      3.570k in   5.010902s
          Oj::Parser    762.598 (± 3.0%) i/s    (1.31 ms/i) -      3.834k in   5.033130s
           rapidjson    553.029 (± 7.4%) i/s    (1.81 ms/i) -      2.750k in   5.022630s

Comparison:
                json:      510.1 i/s
          Oj::Parser:      762.6 i/s - 1.49x  faster
           oj strict:      713.2 i/s - 1.40x  faster
                  oj:      610.2 i/s - 1.20x  faster
           rapidjson:      553.0 i/s - same-ish: difference falls within error

== Parsing citm_catalog.json (1727030 bytes)
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json    28.000 i/100ms
                  oj    33.000 i/100ms
           oj strict    37.000 i/100ms
          Oj::Parser    43.000 i/100ms
           rapidjson    38.000 i/100ms
Calculating -------------------------------------
                json    303.853 (± 3.6%) i/s    (3.29 ms/i) -      1.540k in   5.076079s
                  oj    348.009 (± 2.0%) i/s    (2.87 ms/i) -      1.749k in   5.027738s
           oj strict    396.679 (± 3.3%) i/s    (2.52 ms/i) -      1.998k in   5.042271s
          Oj::Parser    406.699 (± 2.2%) i/s    (2.46 ms/i) -      2.064k in   5.077587s
           rapidjson    393.463 (± 3.3%) i/s    (2.54 ms/i) -      1.976k in   5.028501s

Comparison:
                json:      303.9 i/s
          Oj::Parser:      406.7 i/s - 1.34x  faster
           oj strict:      396.7 i/s - 1.31x  faster
           rapidjson:      393.5 i/s - 1.29x  faster
                  oj:      348.0 i/s - 1.15x  faster
```

After:

```
== Parsing twitter.json (567916 bytes)
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json    56.000 i/100ms
                  oj    62.000 i/100ms
           oj strict    72.000 i/100ms
          Oj::Parser    77.000 i/100ms
           rapidjson    55.000 i/100ms
Calculating -------------------------------------
                json    568.025 (± 2.1%) i/s    (1.76 ms/i) -      2.856k in   5.030272s
                  oj    630.936 (± 1.4%) i/s    (1.58 ms/i) -      3.162k in   5.012630s
           oj strict    705.784 (±11.2%) i/s    (1.42 ms/i) -      3.456k in   5.006706s
          Oj::Parser    783.989 (± 1.7%) i/s    (1.28 ms/i) -      3.927k in   5.010343s
           rapidjson    557.630 (± 2.0%) i/s    (1.79 ms/i) -      2.805k in   5.032388s

Comparison:
                json:      568.0 i/s
          Oj::Parser:      784.0 i/s - 1.38x  faster
           oj strict:      705.8 i/s - 1.24x  faster
                  oj:      630.9 i/s - 1.11x  faster
           rapidjson:      557.6 i/s - same-ish: difference falls within error

== Parsing citm_catalog.json (1727030 bytes)
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json    29.000 i/100ms
                  oj    33.000 i/100ms
           oj strict    38.000 i/100ms
          Oj::Parser    43.000 i/100ms
           rapidjson    37.000 i/100ms
Calculating -------------------------------------
                json    319.271 (± 3.1%) i/s    (3.13 ms/i) -      1.595k in   5.001128s
                  oj    347.946 (± 1.7%) i/s    (2.87 ms/i) -      1.749k in   5.028395s
           oj strict    396.914 (± 3.0%) i/s    (2.52 ms/i) -      2.014k in   5.079645s
          Oj::Parser    409.311 (± 2.7%) i/s    (2.44 ms/i) -      2.064k in   5.046626s
           rapidjson    394.752 (± 1.5%) i/s    (2.53 ms/i) -      1.998k in   5.062776s

Comparison:
                json:      319.3 i/s
          Oj::Parser:      409.3 i/s - 1.28x  faster
           oj strict:      396.9 i/s - 1.24x  faster
           rapidjson:      394.8 i/s - 1.24x  faster
                  oj:      347.9 i/s - 1.09x  faster
```

https://github.com/ruby/json/commit/7e0f66546a

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