fontconfig: enable config files by default

[bmrips]

Description

This PR makes the fonts.fontconfig.configFile.<name>.enable option true by default which matches the behaviour of home.file and the like. Since this change is not backwards-compatible, users with extra config
files are notified of it.

Additionally, I added two tests for the HM-defined config files for font discovery and default fonts.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -A dev --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News