Add .timer & various other systemd extensions (fixes #7161)

[vorburger]

Description

Fixes #7161.

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.automount
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.mount+systemd
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.netdev
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.nspawn
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.socket
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.timer
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have verified that there is no need to include a change to the heuristics to distinguish my language from others using the same extension (because all these new systemd specific file name extensions are not yet used by other other language).

How strictly does this project require "including a real-world usage sample for all extensions added in this PR"? I sure could copy/paste a random example from those "Search results for each extension" linked above, but... what's the point of that? (I'm happy to do so, if it's required to merge this, of course; just wasn't sure if really was a hard-requirement.)

PS: Please note this analysis for which extensions I have opted NOT to include.