parser.c: Record escape positions while parsing

[byroot]

Closes: #894

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 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 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 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

[byroot]

Specific benchmark:

benchmark_parsing "some_unescape", JSON.dump([((" "*100) + "\n")*15] * 100)
benchmark_parsing "more_unescape", JSON.dump([((" "*100) + "\n")*30] * 100)

== Parsing some_unescape (153301 bytes)
ruby 3.4.6 (2025-09-16 revision dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     3.691k i/100ms
Calculating -------------------------------------
               after     36.850k (± 0.5%) i/s   (27.14 μs/i) -    184.550k in   5.008228s

Comparison:
              before:    25529.8 i/s
               after:    36850.3 i/s - 1.44x  faster


== Parsing more_unescape (306301 bytes)
ruby 3.4.6 (2025-09-16 revision dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     1.801k i/100ms
Calculating -------------------------------------
               after     18.039k (± 4.0%) i/s   (55.44 μs/i) -     91.851k in   5.101041s

Comparison:
              before:    14315.5 i/s
               after:    18038.9 i/s - 1.26x  faster

So this helps quite a bit for large strings with a reasonable amount of escapes, typically your comment body, etc, will contain a few newlines and double quotes, but not much more than that.