Improve lookup tables for string escaping.

[byroot]

Introduce a simplified table for the most common case, which is script_safe: false, ascii_only: false.

On the script_safe table, now only 0xE2 does a multi-byte check.

Merge back convert_ASCII_to_JSON, as it no longer help much with the simplified escape table.

== Encoding mixed utf8 (5003001 bytes)
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
               after    38.000 i/100ms
Calculating -------------------------------------
               after    398.220 (± 3.0%) i/s    (2.51 ms/i) -      2.014k in   5.061659s

Comparison:
              before:      381.8 i/s
               after:      398.2 i/s - same-ish: difference falls within error

== Encoding mostly utf8 (5001001 bytes)
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
               after    39.000 i/100ms
Calculating -------------------------------------
               after    393.337 (± 2.5%) i/s    (2.54 ms/i) -      1.989k in   5.059397s

Comparison:
              before:      304.3 i/s
               after:      393.3 i/s - 1.29x  faster

== Encoding twitter.json (466906 bytes)
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
               after   244.000 i/100ms
Calculating -------------------------------------
               after      2.436k (± 0.9%) i/s  (410.43 μs/i) -     12.200k in   5.007702s

Comparison:
              before:     2125.9 i/s
               after:     2436.5 i/s - 1.15x  faster