Add digit separators

[seizethedave]

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

Accompanying issue with format proposal: https://github.com/google/jsonnet/issues/1155

[sbarzowski]

I'm in favor of this. We need to also: 1) Update the docs. At least https://github.com/google/jsonnet/blob/2bca3a02ac3c06e4dac74c10c4b6c650ae53d148/doc/ref/spec.html#L4. 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!