[ruby/json] Refactor convert_UTF8_to_JSON to split searching and escaping code

The goal is to be able to dispatch to more optimized search implementations
without having to duplicate the escaping code.

Somehow, this is a few % faster already:

```
== Encoding activitypub.json (52595 bytes)
ruby 3.4.1 (2024-12-25 revision https://github.com/ruby/json/commit/48d4efcb85) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
               after     2.257k i/100ms
Calculating -------------------------------------
               after     22.930k (± 1.3%) i/s   (43.61 μs/i) -    115.107k in   5.020814s

Comparison:
              before:    21604.0 i/s
               after:    22930.1 i/s - 1.06x  faster

== Encoding citm_catalog.json (500298 bytes)
ruby 3.4.1 (2024-12-25 revision https://github.com/ruby/json/commit/48d4efcb85) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
               after   137.000 i/100ms
Calculating -------------------------------------
               after      1.397k (± 1.1%) i/s  (715.57 μs/i) -      6.987k in   5.000408s

Comparison:
              before:     1344.4 i/s
               after:     1397.5 i/s - 1.04x  faster

== Encoding twitter.json (466906 bytes)
ruby 3.4.1 (2024-12-25 revision https://github.com/ruby/json/commit/48d4efcb85) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
               after   249.000 i/100ms
Calculating -------------------------------------
               after      2.464k (± 1.8%) i/s  (405.81 μs/i) -     12.450k in   5.054131s

Comparison:
              before:     2326.5 i/s
               after:     2464.2 i/s - 1.06x  faster
```

https://github.com/ruby/json/commit/8fb5ae807f

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