Add installers that fetch the newest release when run.

Each installer carries no application of its own and resolves
/releases/latest/download/ at the moment it runs, so the file never goes
stale and the same download keeps working across future releases.

macOS installs into /Applications, falling back to the user's own folder
when that is not writable, and clears the quarantine flag the unsigned
build would otherwise trip over. Windows extracts over any existing
install rather than replacing the folder, so the progress files kept
inside it survive, and adds a Start menu entry. Linux drops the AppImage
in ~/Applications with a desktop entry and its icon.

Publish the macOS one zipped, because a release asset loses its
executable bit and a .command needs one to be double-clicked.

The .cmd also had to be excluded from the *.cmd ignore rule, which comes
from the kernel section of the template and targets build artifacts.

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -63,6 +63,9 @@ assets/icons/
 # Kernel Module Compile Results
 *.mod*
 *.cmd
+# The rule above targets kernel build artifacts such as .foo.o.cmd; the
+# Windows installer is a source file that has to be tracked.
+!installers/*.cmd
 .tmp_versions/
 modules.order
 Module.symvers