Make Ghostty release channel aware

Ghostty now has a release channel build configuration. Current valid
values are "tip" and "stable" but I imagine more will be added in the
future.

The release channel is inferred whether the version we specify with the
`-Dversion-string` build flag has a prerelease tag or not. If it does,
the release channel is "tip". If it doesn't, the release channel is
"stable".

This also adds a configuration to specify the release channel for
auto-updates for the macOS application.

.github/workflows/release-tag.yml

@@ -139,7 +139,11 @@ jobs:
       # GhosttyKit is the framework that is built from Zig for our native
       # Mac app to access. Build this in release mode.
       - name: Build GhosttyKit
-        run: nix develop -c zig build -Doptimize=ReleaseFast
+        run: |
+          nix develop -c \
+            zig build \
+            -Doptimize=ReleaseFast \
+            -Dversion-string=${GHOSTTY_VERSION}
 
       # The native app is built with native XCode tooling. This also does
       # codesigning. IMPORTANT: this must NOT run in a Nix environment.