3.7.2: TOML compliance fix

TOML Fixes

• Implement TOML test suite by @TheElectronWill in #175

The TOML implementation is now more robust thanks to toml-test. We don't call the binary, but rather include the repository as a Git submodule in order to grab all the test files.

Compatibility note: the TOML parser of NightConfig now rejects some files that it would have previously accepted (even though they were invalid). This is intended, and we encourage all users to fix their invalid TOML files.

Please read the release notes of v3.7.0 ✨ to learn what has changed since v3.6.x.

Full Changelog: v3.7.1...v3.7.2