For now, bash integration must be explicitly enabled (by setting
`shell-integration = bash`). Our automatic shell integration requires
bash version 4 or later, and systems like macOS continue to ship bash
version 3 by default. This approach avoids the cost of performing a
runtime version check.
Fish automatic integration taken as an example.
Just like fish, Elvish checks `XDG_DATA_DIRS` for its modules.
Thus, Fish integration in zig is reused, and integration in
Elvish now removes `GHOSTTY_FISH_XDG_DIR` environment variable
on launch.
