Use the pure-Ruby generator on TruffleRuby as it is much faster

[eregon]

• Using the benchmark from https://github.com/flori/json/pull/580

  $ ruby benchmarks/bench.rb dump pure
  JSON::Pure::Generator
  truffleruby 24.0.0, like ruby 3.2.2, Oracle GraalVM Native [x86_64-linux]
  Warming up --------------------------------------
    JSON.dump(obj)   116.000 i/100ms
    JSON.dump(obj)   235.000 i/100ms
    JSON.dump(obj)   317.000 i/100ms
    JSON.dump(obj)   372.000 i/100ms
    JSON.dump(obj)   374.000 i/100ms
  Calculating -------------------------------------
    JSON.dump(obj)      3.735k (± 0.9%) i/s  (267.76 μs/i) -     18.700k in   5.007526s
    JSON.dump(obj)      3.738k (± 0.7%) i/s  (267.49 μs/i) -     18.700k in   5.002252s
    JSON.dump(obj)      3.743k (± 0.7%) i/s  (267.18 μs/i) -     19.074k in   5.096375s
    JSON.dump(obj)      3.747k (± 0.5%) i/s  (266.87 μs/i) -     19.074k in   5.090463s
    JSON.dump(obj)      3.746k (± 0.5%) i/s  (266.96 μs/i) -     19.074k in   5.092069s
  $ ruby benchmarks/bench.rb dump ext
  JSON::Ext::Generator
  truffleruby 24.0.0, like ruby 3.2.2, Oracle GraalVM Native [x86_64-linux]
  Warming up --------------------------------------
    JSON.dump(obj)    19.000 i/100ms
    JSON.dump(obj)    18.000 i/100ms
    JSON.dump(obj)    18.000 i/100ms
    JSON.dump(obj)    18.000 i/100ms
    JSON.dump(obj)    21.000 i/100ms
  Calculating -------------------------------------
    JSON.dump(obj)    221.260 (±10.8%) i/s    (4.52 ms/i) -      1.092k in   5.004381s
    JSON.dump(obj)    221.983 (± 8.1%) i/s    (4.50 ms/i) -      1.113k in   5.055574s
    JSON.dump(obj)    221.446 (± 8.6%) i/s    (4.52 ms/i) -      1.113k in   5.073167s
    JSON.dump(obj)    226.452 (± 7.9%) i/s    (4.42 ms/i) -      1.134k in   5.048568s
    JSON.dump(obj)    227.795 (± 8.3%) i/s    (4.39 ms/i) -      1.134k in   5.025187s

[eregon]

@hsbt Could you review this too? (if not, who should I ask?)

(I rebased to make sure it runs the CI with the truffleruby jobs too)

[hsbt]

I'm super busy now for Ruby development and RubyKaigi. I have a time after middle of May.

Please wait until that time.