Tree-sitter grammar: support Constant Declaration

[OceanOak]

Changelog:

Tree-sitter-darklang
- Update the grammar to support Constant Declaration

#5321

[StachuDotNet]

I would expect this to be much simpler: rather than reinvent each grammar part, the RHS of a const declaration should be a normal Expression - and in the parser side of things (or maybe later, even), that's where we could error on trying to set an 'illegal' (unsupported) constant.

Edit: Hmm - I need to think about this a bit more. In ProgramTypes, we have separate things for each legal constant option. Maybe this is OK, or maybe we should make PT a bit simpler 🤔

[StachuDotNet]

Great! Btw, I think 'typesAndFnsAndConsts' could be renamed to 'declarations' (incl when it includes handler definitions)