Flatpak for the Helix editor

To update:

• Clone the helix repo and checkout desired version

  git clone https://github.com/helix-editor/helix
  git -C helix switch --detach 24.07

• Use flatpak-cargo-generator.py to generate cargo-sources.json from helix/Cargo.lock:

  python flatpak-cargo-generator.py helix/Cargo.lock -o cargo-sources.json

• Use grammars/parse_languages.py to generate grammars/grammar-sources.json from helix/languages.toml:

  python grammars/parse_languages.py ./helix/languages.toml

• Change url and checksum of helix source code in com.helix_editor.Helix.yml

• Build and locally install the flatpak:

  flatpak-builder build-dir com.helix_editor.Helix.yml --user --install --force-clean --ccache