Offer better control over "languageid" value sent to lsp

Offer better control over "languageid" value sent to lsp

Handles the issue of languages whose major mode has a
different name than the name that the LSP server expects for
the language.  One can now:

(put 'favourite-major-mode 'eglot-language-id "foobarbaz")

And "foobarbaz" will be used as the LSP "languageId" value.

* eglot.el (eglot--TextDocumentItem): Consult 'eglot-language-id.

Copyright-paperwork-exempt: yes
GitHub-reference: fix https://github.com/joaotavora/eglot/issues/525