Add digit separators

[seizethedave]

This PR adds digit separators (1_000) to Jsonnet's numeric constants.

Accompanying issue with format proposal: #1155

[sbarzowski]

I'm in favor of this. We need to also:

1. Update the docs. At least

   jsonnet/doc/ref/spec.html

   Line 4 in 2bca3a0

   ---

   . Ideallly also the tutorial.

See https://github.com/google/jsonnet?tab=readme-ov-file#locally-serving-the-website for working on documentation.

2. Add some end-to-end examples. Just dump some .jsonnet files in test_suite and run https://github.com/google/jsonnet/blob/master/test_suite/refresh_golden.sh.

3. We'll need to update go-jsonnet implementation (separate PR). It should be straightforward.

[seizethedave]

Thanks @sbarzowski, I'll get cracking on those.

[seizethedave]

@sbarzowski I think I'm ready for another round of feedback on this one. I suspect these might need a little more help:

• a more formal treatment of the numeric grammar in the docs
• note how std.parseInt and related functions will not honor numbers with underscores
  Thanks!

[seizethedave]

Anything I can do to push this forward @sbarzowski? Thanks!