Add .helix to .gitignore

[helix] can use project specific configuration in the `.helix` directory under
the project root. For gst-rs development, this can be used to select a group of
features:

.helix/languages.toml:

```toml
[language-server.rust-analyzer.config]
cargo = { features = ["v1_26"] }
```

It can also be used to configure debugging targets, thought debugging experience
with helix is lacking currently.

[helix]: https://helix-editor.com/

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1554>

.gitignore

@@ -1,2 +1,3 @@
 target/
 **/*.rs.bk
+.helix/