[ruby/json] parser.c: Record escape positions while parsing

We can then pass them to the decoder to save having to parse
the string again.

```
== Parsing activitypub.json (58160 bytes)
ruby 3.4.6 (2025-09-16 revision https://github.com/ruby/json/commit/dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     1.275k i/100ms
Calculating -------------------------------------
               after     12.774k (± 0.8%) i/s   (78.29 μs/i) -     65.025k in   5.090834s

Comparison:
              before:    12314.3 i/s
               after:    12773.8 i/s - 1.04x  faster

== Parsing twitter.json (567916 bytes)
ruby 3.4.6 (2025-09-16 revision https://github.com/ruby/json/commit/dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after   143.000 i/100ms
Calculating -------------------------------------
               after      1.441k (± 0.2%) i/s  (693.86 μs/i) -      7.293k in   5.060345s

Comparison:
              before:     1430.1 i/s
               after:     1441.2 i/s - 1.01x  faster

== Parsing citm_catalog.json (1727030 bytes)
ruby 3.4.6 (2025-09-16 revision https://github.com/ruby/json/commit/dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after    69.000 i/100ms
Calculating -------------------------------------
               after    695.919 (± 0.4%) i/s    (1.44 ms/i) -      3.519k in   5.056691s

Comparison:
              before:      687.8 i/s
               after:      695.9 i/s - 1.01x  faster
```

https://github.com/ruby/json/commit/4f4551f993

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