[ruby/json] Optimize convert_UTF8_to_JSON for mostly ASCII strings

If we assume that even UTF-8 strings are mostly ASCII, we can implement a
fast path for the ASCII parts.

Before:

```
== Encoding mixed utf8 (20012001 bytes)
ruby 3.4.0dev (2024-10-18T15:12:54Z master https://github.com/ruby/json/commit/d1b5c10957) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json     5.000 i/100ms
                  oj     9.000 i/100ms
           rapidjson     2.000 i/100ms
Calculating -------------------------------------
                json     49.403 (± 2.0%) i/s   (20.24 ms/i) -    250.000 in   5.062647s
                  oj    100.120 (± 2.0%) i/s    (9.99 ms/i) -    504.000 in   5.035349s
           rapidjson     26.404 (± 0.0%) i/s   (37.87 ms/i) -    132.000 in   5.001025s

Comparison:
                json:       49.4 i/s
                  oj:      100.1 i/s - 2.03x  faster
           rapidjson:       26.4 i/s - 1.87x  slower
```

After:

```
== Encoding mixed utf8 (20012001 bytes)
ruby 3.4.0dev (2024-10-18T15:12:54Z master https://github.com/ruby/json/commit/d1b5c10957) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json    10.000 i/100ms
                  oj     9.000 i/100ms
           rapidjson     2.000 i/100ms
Calculating -------------------------------------
                json     95.686 (± 2.1%) i/s   (10.45 ms/i) -    480.000 in   5.018575s
                  oj     96.875 (± 2.1%) i/s   (10.32 ms/i) -    486.000 in   5.019097s
           rapidjson     26.260 (± 3.8%) i/s   (38.08 ms/i) -    132.000 in   5.033151s

Comparison:
                json:       95.7 i/s
                  oj:       96.9 i/s - same-ish: difference falls within error
           rapidjson:       26.3 i/s - 3.64x  slower
```

https://github.com/ruby/json/commit/f8166c2d7f

0 files changed, 0 insertions, 0 deletions