Add minimal fontconfig support

[PgBiel]

This PR intends to bring the upstream fixes for fontconfig support, in typst/typst#2472, to the Typst LSP. This is necessary for proper font lookup support on NixOS, and is done by using the fontdb package to search for fonts. I also added a feature to toggle fontconfig support (and thus revert to the old behavior) if necessary (in case some Linux distribution isn't compatible with it or something).

Fixes #358.

Note: This does not seem to work perfectly when using VSCode through Flatpak together with NixOS' Home Manager, but setting the environment variable FONTCONFIG_FILE to point to the relevant .conf file (e.g. FONTCONFIG_FILE="/home/username/.config/fontconfig/conf.d/10-hm-fonts.conf") seems to fix it. (When using VSCode outside Flatpak, that is not at all necessary, as long as this PR's patches are applied. Note that such behavior is not the LSP's fault; it appears to just be a limitation within fontdb.)

(Edit: possibly relevant upstream issue: RazrFalcon/fontdb#59)

I added a note about this to the README. Ideally, we'd have support for custom font paths as an option in the LSP itself to remedy this in a more general manner, but this will suffice for now.

[nvarner]

@PgBiel can you add the new feature to the native-tls steps in .github/workflows/checks.yml?

Otherwise (assuming checks pass), this looks good, thanks! I appreciate the documentation.

[PgBiel]

So, VSCode automatically removed trailing spaces in the workflow files, so I added that change in a separate commit. I updated checks.yml, and also saw that there was a test in full_build.yml that could perhaps use an update as well, so I changed it in a separate commit in case you think that's not appropriate and should be reverted (feel free).

[nvarner]

The formatting changes are fine. Thank you!