fix: add .tool-versions in gitignore

The tool that requries this file (called "asdf")
is suggested in the AppFlowy's documentation for
installing Flutter and Dart, so it makes sense to
have it ignored by default.

Maybe it should be committed,
so it whould be easier to keep all contributors with the same Flutter version.

.gitignore

@@ -40,3 +40,4 @@ frontend/package
 frontend/*.deb
 
 **/Cargo.toml.bak
+.tool-versions