fix: update grammar to support user defined imports

[cronsorcery]

Hi.

I have encountered a few issues while using this parser with experimental features.

Since 0.23.x bicep supports imports of user-defined functionality.
There's also provider syntax suggested by bicep-lsp, which is supposed to behave like import statements.

Using this feature breaks highlighting, since tree-sitter is looking for a version string, and resource declaration looks suspiciously close to it:

This PR introduces a few tweaks to existing imports to support provider and adds import_user_functionality as described in official documentation:

Not sure where to find docs for provider, but both LSP and bicep seem to not mind it at all.
Tested on Bicep CLI version 0.23.1 (b02de2da48).

Some additional work for the future:

• nullable types (e.g.: type foo = string?) shouldn't cause errors
• user defined functions
• multiline strings with extra unescaped quotes should be valid

[amaanq]

Thanks for the PR!

For the multiline string with unescaped quotes, that will probably need to be parsed in the scanner
For the nullable types, some docs on that would be great

I will try and implement those missing features later on when I get time, but if you'd like to take a stab at it go ahead and I can help out when needed.

[amaanq]

Alright, I've implemented all those features as well as moving multiline string token parsing to the external scanner. Enjoy and let me know of other issues you run into!

[cronsorcery]

Much appreciated, thank you. I'll make sure to do so or see if I can help improve it myself.