[ruby/json] Allocate the initial generator buffer on the stack

Ref: https://github.com/ruby/json/issues/655
Followup: https://github.com/ruby/json/issues/657

Assuming the generator might be used for fairly small documents
we can start with a reasonable buffer size of the stack, and if
we outgrow it, we can spill on the heap.

In a way this is optimizing for micro-benchmarks, but there are
valid use case for fiarly small JSON document in actual real world
scenarios, so trashing the GC less in such case make sense.

Before:

```
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                  Oj   518.700k i/100ms
          JSON reuse   483.370k i/100ms
Calculating -------------------------------------
                  Oj      5.722M (± 1.8%) i/s  (174.76 ns/i) -     29.047M in   5.077823s
          JSON reuse      5.278M (± 1.5%) i/s  (189.46 ns/i) -     26.585M in   5.038172s

Comparison:
                  Oj:  5722283.8 i/s
          JSON reuse:  5278061.7 i/s - 1.08x  slower
```

After:

```
ruby 3.3.4 (2024-07-09 revision https://github.com/ruby/json/commit/be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
                  Oj   517.837k i/100ms
          JSON reuse   548.871k i/100ms
Calculating -------------------------------------
                  Oj      5.693M (± 1.6%) i/s  (175.65 ns/i) -     28.481M in   5.004056s
          JSON reuse      5.855M (± 1.2%) i/s  (170.80 ns/i) -     29.639M in   5.063004s

Comparison:
                  Oj:  5692985.6 i/s
          JSON reuse:  5854857.9 i/s - 1.03x  faster
```

https://github.com/ruby/json/commit/fe607f4806

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