feat(shell-integration): add automatic integration for Elvish

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.

src/config/Config.zig

@@ -3414,6 +3414,7 @@ pub const ShellIntegration = enum {
     bash,
     fish,
     zsh,
+    elvish,
 };
 
 /// Shell integration features