[tree-sitter] add variable identifier support in assignments expressions

[alxtz]

Description of the Change

Originally, variables created from assignments aren't given the .syntax--variable class, which makes the original language-ruby package not able to provide any type of way to style a variable.

Alternate Designs

Some grammar package would also add .syntax--other for this kind of variable, can add if needed.

Applicable Issues

#252 Adding more support for the new used tree-sitter parsing system

[rsese]

Thanks @alxtz! Since c616809#diff-336e0e422308721e5f4dc0cdc311417f, tests have been added for Tree-sitter grammar changes - are you up for adding tests to help prevent future regressions?

[alxtz]

Hi @rsese I've added the corresponding happy path & sad path test for this, can you take a look?

55ab414

[rsese]

Awesome, thanks @alxtz ✨ I wouldn't be the person reviewing the pull request, just checking for the existence of tests at this point - we can't promise a specific timeframe for review but if there are any followup questions about the tests or anything else, whoever ends up reviewing will comment here in the PR.