Fix fonts on NixOS and any other XDG-compliant distro with unusual directory structure

[easrng]

NixOS doesn't have /usr/share/fonts, so Ladybird was only loading the default bitmap font. This fixes that by replacing hardcoded paths (/usr/share/fonts, /usr/local/share/fonts, $HOME/.local/share/fonts) with their XDG base directory equivalents (with fallbacks included so nothing changes on systems where $XDG_DATA_HOME or $XDG_DATA_DIRS are unset). It also silences the warnings when nonexistent directories are returned from StandardPaths.cpp::font_directories(), since that is to be expected with the updated code that checks for more possible directories.

[ADKaster]

Whoops, looks like the code I gave you, or the edits you made (either or) are now causing a use after free https://github.com/LadybirdWebBrowser/ladybird/actions/runs/9357541272/job/25757496121?pr=31#step:14:47

Do feel free to resolve resolved comments yourself btw, the only people who can do that on github are the author and people with write access.

[easrng]

the tests pass locally so idk how to fix this with no debug feedback... trying a rebuild with CC=clang CXX=clang++

[ADKaster]

I haven't quite updated it yet, but there is a RunningTests.md documentation file. Basically, poke CMake to -DENABLE_UNDEFINED_SANITIZER=ON and -DENABLE_ADDRESS_SANITIZER=ON and set the associated OPTIONS environment variables to reproduce the CI failure

[fgaz]

fyi this was temporarily fixed downstream by hardcoding the nixos font path in NixOS/nixpkgs#315048

[ADKaster]

The bug was that you had

Core::Environment::get("XDG_DATA_DIRS"_string).value_or("/usr/local/share:/usr/share"_string);

However, Core::Environment::get() returns Optional<StringView>. So when XDG_DATA_DIRS was not defined, we use-after-free'd the temporary string in the value_or branch.

The fix is straightforward: Don't use temporary strings, use stringview literals

Core::Environment::get("XDG_DATA_DIRS"sv).value_or("/usr/local/share:/usr/share"sv);

I'll push this tiny fix to your branch and merge it soon