feat(text-editor): add support for `underline` formatting

[john-traas]

We're unsure as to how to go about handling underline with markdown as it is not supported.

Probably we'll need to have conditional availability on the menu item for underline, but this is not part of MVP. Customisable menu items is on the wishlist from addons. We can tackle implementation of this at the same time.

Review:

• [ ] Commits are atomic
• [ ] Commits have the correct type for the changes made
• [ ] Commits with breaking changes are marked as such

Browsers tested:

(Check any that applies, it's ok to leave boxes unchecked if testing something didn't seem relevant.)

Windows:

• [ ] Chrome
• [ ] Edge
• [ ] Firefox

Linux:

• [ ] Chrome
• [ ] Firefox

macOS:

• [ ] Chrome
• [ ] Firefox
• [ ] Safari

Mobile:

• [ ] Chrome on Android
• [ ] iOS