PR: add support for 3rd-party font providers

[o-murphy]

Summary

Adds a plugin system that lets third-party packages register extra iconic fonts with qtawesome, without needing changes on the qtawesome side.

• qtawesome/__init__.py: _get_external_fonts() discovers packages that expose an entry point in the qtawesome.font_providers group (via importlib.metadata.entry_points), calls each one, and loads the returned fonts alongside the bundled ones. Falls back to the pre-3.10 entry_points() API for Python 3.9 compatibility.
• font_provider_template/: reference implementation showing how to build a provider package - entry point registration in pyproject.toml, a get_fonts() callable returning (prefix, ttf_filename, charmap_filename, directory) tuples, and a working example bundling the Simple Icons font (si prefix) under its own CC0 1.0 license, separate from the template's MIT license.

Note: this is a proposal for the concept. The project's main documentation has not been updated yet - it will only be updated once the approach is agreed upon, to avoid documenting an API that might still change during review.

font_provider_template/ currently lives inside this repo for review convenience; once the concept is agreed upon, it could be moved out to its own repository (or a dedicated branch) so that anyone wanting to build a font provider can start from it directly, without cloning qtawesome itself.

Test plan

git clone https://github.com/o-murphy/qtawesome -b feat/font_providers
cd qtawesome
python3 -m venv .venv
. .venv/bin/activate
pip install -e .
pip install -e font_provider_template
qta-browser  # the 'si' prefix is now available

Relative to: #310