chore(packaging): add AUR package sources for source, git, and binary builds

packaging/aur/ now holds the full content of three AUR repositories:
proton-drive-for-linux builds the latest tag from source,
proton-drive-for-linux-git builds main, and proton-drive-for-linux-bin
installs the binaries attached to the GitHub release. Each carries a
PKGBUILD, a generated .SRCINFO, an ignore-everything .gitignore, and the
0BSD license the AUR expects for package sources.

update.sh bumps the release-based packages and regenerates the .SRCINFO
files; push.sh publishes each package to its AUR remote. The README now
points users at the AUR packages and keeps the in-tree PKGBUILD as the
development build.

packaging/aur/proton-drive-for-linux-git/.gitignore

@@ -0,0 +1,7 @@
+# Keep the AUR repository to the sources only: ignore everything, then
+# force-add the tracked files with `git add -f`.
+*
+!.gitignore
+!.SRCINFO
+!LICENSE
+!PKGBUILD