bug#66183: elixir-ts-mode test failure

[Yuan Fu]

> On Sep 24, 2023, at 9:53 AM, john muhl via Bug reports for GNU Emacs, the 
> Swiss army knife of text editors <bug-gnu-emacs@gnu.org> wrote:
> 
> The change in 5cba5ee8905 caused an elixir test to start failing. The
> white space at the start of the test is now in a heex range so pressing
> tab there matches a heex rule instead of the elixir one. Moving the
> dummy range to the end fixes the test.

Thank you. I’ll make sure to remember also running elixir tests when I make a 
change.

I looked around and found elixir-mode’s language-at-point function works in a 
way that isn’t what tree-sitter functions expect. Tree-sitter functions expect 
the major mode to derive language at point by querying the host language and do 
some pattern matching, rather than using language parser’s range. I took a 
quick look at elixir’s grammar and came up with this version for a POC. You 
probably need to adjust it but the idea is there.

With the new language-at-point definition, treesit-language-at can return the 
correct language, and the test passes again.

Yuan

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