Add systemd-language-server to implementations

[psacawa]

Description (e.g. "Related to ...", etc.)

Add systemd-language-server to LSP implementations using pygls

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

[tombh]

Thank you! What a great LSP server 🤓

I can see that your commit message will need to follow the Conventional Commits syntax, so something like, docs: add systemd-language-server to implementations.

[psacawa]

I have implemented this change