LSP plugin: Use language id instead of highlighting mode for server identification

Summary:
Use the standardized language id in more places.

Why?
- the LSP names are "standardized"
- we can internally map different C++/... highlightings to the same canonical LSP lanuage id name

Reviewers: mnauwelaerts, dhaumann

Reviewed By: mnauwelaerts, dhaumann

Subscribers: dhaumann, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D23009