Support additional LaTeX commands and environments in ltex-ls

[m-haug]

Is your feature request related to a problem? Please describe.

I'm using ltex-ls with LaTeX and the csquotes package in a German document.
When using the csquotes command \foreignquote to insert quoted text in a foreign language, i.e., \foreignquote{british}{Some English text here}, I get diagnostics from ltex-ls in the English text.
The diagnostic messages indicate that ltex-ls does not recognize the language change and is still applying the German ruleset.

Describe the solution you'd like

I would like ltex-ls to recognize the language changing commands and environments in the csquotes package.

Describe alternatives you've considered

I recognize that it may not be feasible for ltex-ls to support individual packages.
As an alternative, I think my use case, and similar use cases, would be fulfilled by configuration options to add additional language changing commands and environments to ltex-ls.
These could work by specifying a command and either a hardcoded or dynamic language to switch to inside a text argument.

For now, I'm working around the issue by using the roughly equivalent command sequence \foreignlanguage{british}{\enquote{Some English text here}}.
However, the nested commands are not as nice as using a single command to do both things.