parser.c: optimize json_string_unescape

[samyron]

This PR optimizes json_string_unescape.

Two commits:

1. Use ARM Neon to scan for \. While scanning, copy the current chunk to the output.
2. Add a fast path when unescaping a single character.

If this PR is accepted, I will follow up with an SSE2 implementation.

Benchmarks

Run on a Macbook Air M1.

twitterescaped.json is from simdjson-data.

== Parsing activitypub.json (58160 bytes)
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     1.103k i/100ms
Calculating -------------------------------------
               after     11.143k (± 0.8%) i/s   (89.74 μs/i) -     56.253k in   5.048516s

Comparison:
              before:    10366.8 i/s
               after:    11143.2 i/s - 1.07x  faster

== Parsing twitterescaped.json (562408 bytes)
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after    73.000 i/100ms
Calculating -------------------------------------
               after    737.341 (± 0.9%) i/s    (1.36 ms/i) -      3.723k in   5.049667s

Comparison:
              before:      712.1 i/s
               after:      737.3 i/s - 1.04x  faster

I should note that the fast path for unescaping a single character accounts for about 1% of the speed increase in activitypub.json. It's pretty minor.