feat: add lfortran as an LSP client

[gnikit]

Is there an existing request for this?

• I have searched the existing issues

Feature Request

With the LSP functionality quickly advancing in LFortran we should include it as an alternative server in the extension.
The only thing required is to implement lfortran's LSP API and add the option in our LSP client registration.

This is in addition to including lfortran as a linter, see #589

Some refactoring and renaming might be in order since classes and objects are named with fortls but in reality are just generic options.

cc @certik