Add LanguageID to file

[Nepomuceno]

Adding the language id so we could distinguish between tfvars and terraform files

This will assist the implementation for #50 and #519.

[radeksimko]

This looks good at first sight - can you also try wiring it up to didOpen handler now and maybe write a small integration test which checks that the languageID is stored?

See the existing test in https://github.com/hashicorp/terraform-ls/blob/main/internal/langserver/handlers/did_open_test.go for inspiration. You can create your own instance of the filesystem.Filesystem via fileystem.NewFilesystem(), pass it to the mock server and then check the languageID stored via GetDocument() & your new LanguageID() method.

Then I'd be happy to merge it - as storing language ID (although we wouldn't do anything with it yet) looks like a good-sized deliverable.

[Nepomuceno]

Added some test and hopefully fix the build
Of course that was not true ;) but now I think I fixed the build

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.