feat(css): Adding GitLab Sans & JetBrains Mono as new default fonts

-   Adds GitLab Sans & JetBrains Mono to the list of fonts used.
    Serving fonts in a self-hosting approach. The fonts are distributed
    via the `@gitlab/fonts` npm package

-   Rename `typography.scss` to `fonts.sccss`

    This is done to avoid confusion with `typography.scss`
    in `utility-mixins/typography.scss`

-   Mark font variables as `!default`

    This allows to overwrite them in downstream projects.

BREAKING CHANGE: remove ` gl-font-size-monospace-(size)` utilities
Instead use standard `gl-font-(size)`

Co-authored-by: Olena Horal-Koretska <ohoralkoretska@gitlab.com>
Co-authored-by: Lukas Eipert <leipert@gitlab.com>