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.
If the apprt is GTK add the GTK and the libadwaita version info to the
`+version` action. This information is available in the log but it may
be more accessible here.
Doc strings attached to fields of the Config struct and doc strings
attached to the run function of actions will be used to generate Zig
code that makes those doc strings available to be used at runtime.
Based on PR #853 by @Raiden1411
