root: add .helix to .gitignore

[helix] can use project specific configuration in the `.helix` directory under
the project root. For GStreamer development, this can be used to:

* point the lsp (clangd) to the folder where `compile_commands.json` is
  generated,
* avoid automatic header insertion,
* define gst-indent as the formatter (from the gstreamer/scripts dir).

.helix/languages.toml:

```toml
[language-server.clangd]
args = [
  "--compile-commands-dir=builddir",
  "--header-insertion=never",
]

[[language]]
name = "c"
auto-format = true
formatter = { command = "gst-indent", args = ["-st"] }
```

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

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7612>

.gitignore

@@ -6,6 +6,7 @@ prefix/
 .gdbinit
 
 # Editor/IDE spcific
+.helix/
 .vscode/
 .vscode-server/