feat: add .desktop, log files, systemd units and Gettext PO

[kisaragi-hiu]

I grouped these languages into one PR, because otherwise I would be about to open literally a dozen PRs.

The generated / converted / fetched files are added in a separate commit to make it easier to review.

• I chose https://github.com/nico-castell/desktop-file-support for .desktop files. These are similar to INI but have extra keywords (as well as the Key[lang]=value syntax) that make highlighting them as INI a little clumsy.
• Gettext PO is stable enough that I used TextMate's grammar.
• I chose https://github.com/hangxingliu/vscode-systemd/ for the systemd unit grammar. Systemd unit syntax is inspired by .desktop, but also extends the syntax by adding line continuations, so using a dedicated grammar is also justified.
• Log files: a grammar for them is included in VSCode but it isn't in Shiki and it would be better if they're added. As the grammar file indicates that it tracks an upstream version, I've added the upstream (and placed it in sourcesCommunity) instead.

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