[ruby/json] convert_UTF8_to_JSON: repurpose the escape tables into size tables

Since we're looking up the table anyway, we might as well store the
UTF-8 char length in it. For single byte characters that don't need
escaping we store `0`.

This helps on strings with lots of multi-byte characters:

Before:

```
== Encoding mostly utf8 (20004001 bytes)
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json     6.000 i/100ms
                  oj    10.000 i/100ms
           rapidjson     2.000 i/100ms
Calculating -------------------------------------
                json     67.978 (± 1.5%) i/s   (14.71 ms/i) -    342.000 in   5.033062s
                  oj    100.876 (± 2.0%) i/s    (9.91 ms/i) -    510.000 in   5.058080s
           rapidjson     26.389 (± 7.6%) i/s   (37.89 ms/i) -    132.000 in   5.027681s

Comparison:
                json:       68.0 i/s
                  oj:      100.9 i/s - 1.48x  faster
           rapidjson:       26.4 i/s - 2.58x  slower
```

After:

```
== Encoding mostly utf8 (20004001 bytes)
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                json     7.000 i/100ms
                  oj    10.000 i/100ms
           rapidjson     2.000 i/100ms
Calculating -------------------------------------
                json     75.187 (± 2.7%) i/s   (13.30 ms/i) -    378.000 in   5.030111s
                  oj     95.196 (± 2.1%) i/s   (10.50 ms/i) -    480.000 in   5.043565s
           rapidjson     25.969 (± 3.9%) i/s   (38.51 ms/i) -    130.000 in   5.011471s

Comparison:
                json:       75.2 i/s
                  oj:       95.2 i/s - 1.27x  faster
           rapidjson:       26.0 i/s - 2.90x  slower
```

https://github.com/ruby/json/commit/51e2631d1f

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