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

[vorburger]

Description

Fixes #7161.

Checklist:

• [X] I am adding a new extension to a language.
  • [X] 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):
  • [X] 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.

[vorburger]

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.)

It is. They're used to train the classifier.

ACK, thanks; I'll add samples.

Also every filename and extension needs to meet our usage requirements. One lesser used file or extension will hold up the PR until it meets the usage requirements.

Do you prefer separate small PRs - one for each?

[lildude]

Do you prefer separate small PRs - one for each?

One PR is fine, and be sure to only include extensions that meet our usage requirements as the PR will be held up waiting for any that don't meet our requirements.

[vorburger]

We def don't need the comments

Inline comments now removed.

and only really want files and extensions which meet our usage requirements

These do, do the best of my understanding of your criteria, and knowledge. If I misunderstood, let me know, and I'll remove.

See https://github.com/github-linguist/linguist/issues/7161#issuecomment-2543120512 for which extensions I have opted NOT to include.

along with a sample of each

Samples now added.