Ignore local environment .tool-versions configuration file (#38013)

The `.tool-versions` file configures one's local environment and should
likely not be committed. Its format is defined by
[asdf-vm](https://asdf-vm.com), and is similar to
[nvm](https://github.com/nvm-sh/nvm), [rvm](https://rvm.io), or
[rbenv](https://github.com/rbenv/rbenv). Ignore this file will help
ensure the configuration is not accidentally committed to the
repository.

.gitignore

@@ -29,3 +29,4 @@ test/native/junit.xml
 phpcs.xml
 phpunit.xml
 phpunit-watcher.yml
+.tool-versions