Update CI to test against PHP 8.0 - 8.2

Make RegExMatch compatible with PHP 8.1 (#94)

Drop `mbstring` as a hard dependency (#92)

Default to using `strtolower` and only use `mb_strtolower` when it is
available and necessary. This provides a minor speed boost and allows
running on environments without the `mbstring` extension is not
installed.

Merge pull request #91 from scrivo/chore/utf8-tests

Remove 'ext-dom' from suggested

Merge pull request #84 from scrivo/hotfix/regex-performance

Update docs for new utility functions

Merge pull request #80 from scrivo/hotfix/split-code-empty-string-war…

…ning

Exit out early when empty string is split

Fix UTF-8 support in splitCodeIntoArray() (#76)

Merge pull request #69 from scrivo/feature/theme-bg-colors

Introduce new `getThemeBackgroundColor` utility function