Total rewrite, better syntax, 15 new assertions, actual documentation

What's New?

Rewrote the library from scratch! It's not a monkeypatch on top of Lust anymore.
New settings:
- thirst.is_coverage_bar_enabled;
- thirst.is_printing_enabled;
- thirst.is_print_errors_on_finish_enabled.
New assertions:
- expect.pass() and expect.fail();
- expect.does_not_exist();
- expect.not_equals();
- expect.is_a() and expect.is_not_a();
- expect.function_fails();
- expect.contains() and expect.does_not_contain();
- expect.is_empty() and expect.is_not_empty();
- expect.greater_than(), expect.greater_than_or_equal_to(), expect.lesser_than() and expect.lesser_than_or_equal_to().
Now includes Lust's before(), after() and spy() functionalities.
run_folder() lets you execute your whole test suite at once (with LÖVE).
Now has unit tests.

Changes

Doesn't use to_fail() to invert tests anymore, as it was too confusing. Instead, most assertions will have an opposite version, like is_a() and is_not_a().
nocolor() has been replaced with is_color_enabled.
describe(), _push() and _pop() are now section(), push_section() and pop_section().
_new_assertion() has been replaced with create_assertion().

expect.has_substring(), expect.match(), maybe expect.starts_with() and expect.ends_with().

All in all, that was over 1600 lines changed, with 1.2k additions and ~500 deletions. Whew!