[Fairground 🎡] Add expanded nav components

[frederickobrien]

This adds a new suite of TitlePieceExpandedNav components as part of the ongoing masthead work. After going down the wrong road for a while I've wound up replicating the old Nav components to amend and build on, rather than trying to rebuild the whole thing from scratch.

As part of making these new components (and Storybook stories) I've also created a /masthead directory for the various parts of Fairground's work to go. As a dev new to the repository I found it much easier to work in than a huge, flat approach.

Things not included in this PR but to be implemented in follow-ups:

• [ ] Expanded nav alignment and styling
• [ ] Veggie burger toggle for reactive behaviour and animations
• [ ] Functioning search bar in desktop expanded nav A placeholder search bar with basic styling is included here but implementation will have to wait

Plus bugs we haven't had the pleasure of finding yet.

Screenshots

(There's not much to see here yet as the new components look much the same as the old ones. Updated designs are being implemented in #11691)