fix(deps): replace optional dependencies with dependency groups

Definition: https://packaging.python.org/en/latest/specifications/dependency-groups

The optional dependency table ([project.optional-dependencies]) is
supposed to be user-facing for installs of TagStudio. Meanwhile, the
replacement dependency groups is explicitly meant for extra development
tools, so lets use that.

This does mean the command for a full development environment has changed
from:
```sh
pip install -e '.[dev]'
```

To:
```sh
pip install -e . --group all
```

The documentation has been adjusted for a quick replacement, but
probably should have more information on a later time.

nix/shell.nix

@@ -120,7 +120,7 @@ pkgs.mkShellNoCC {
 
       if [ ! -f "''${venv}"/pyproject.toml ] || ! diff --brief pyproject.toml "''${venv}"/pyproject.toml >/dev/null; then
           printf '%s\n' 'Installing dependencies, pyproject.toml changed...' >&2
-          uv pip install --quiet --editable '.[mkdocs,mypy,pre-commit,pytest]'
+          uv pip install --quiet --editable . --group docs --group extra --group test
           cp pyproject.toml "''${venv}"/pyproject.toml
       fi