Add New Tab Menu Customization to Settings UI (#18015)

## Summary of the Pull Request
Adds customization for the New Tab Menu to the settings UI.

- Settings Model changes:
- The Settings UI generally works by creating a copy of the entire
settings model objects on which we apply the changes to. Turns out, we
completely left the NewTabMenu out of that process. So I went ahead and
implemented it.
   -  `FolderEntry`
- `FolderEntry` exposes `Entries()` (used by the new tab menu to figure
out what to actually render) and `RawEntries()` (the actual JSON data
deserialized into settings model objects). I went ahead and exposed
`RawEntries()` since we'll need to apply changes to it to then
serialize.
- View Model:
- `NewTabMenuViewModel` is the main view model that interacts with the
page. It maintains the current view of items and applies changes to the
settings model.
- `NewTabMenuEntryViewModel` and all of the other `_EntryViewModel`
classes are wrappers for the settings model NTM entries.
- `FolderTreeViewEntry` encapsulates `FolderEntryViewModel`. It allows
us to construct a `TreeView` of just folders.
- View changes and additions:
   - Added FontIconGlyph to the SettingContainer
   - Added a New Tab Menu item to the navigation view
- Adding entries: a stack of SettingContainers is used here. We use the
new `FontIconGlyph` to make this look nice!
- Reordering entries: drag and drop is supported! This might not work in
admin mode though, and we can't drag and drop into folders. Buttons were
added to make this keyboard accessible.
- To move entries into a folder, a button was added which then displays
a TreeView of all folders.
   - Multiple entries can be moved to a folder or deleted at once!
   - Breadcrumbs are used for folders
- When a folder is entered, additional controls are displayed to
customize that folder.
 
## Verification
- ✅ a11y pass
- ✅ keyboard accessible
- scenarios:
   - ✅ add entries (except actions)
   - ✅ changes propagated to settings model (aka "saving works")
   - ✅ reorder entries
   - ✅ move entries to an existing folder
   - ✅ delete multiple entries
   - ✅ delete individual entries
   - ✅ display entries (including actions)

## Follow-ups
- [ ] add support for adding and editing action entries
- [ ] when we discard changes or save, it would be cool if we could stay
on the same page
- [ ] allow customizing the folder entry _before_ adding it (current
workaround is to add it, then edit it)
- [ ] improve UI for setting icon (reuse UI from #17965)

.github/actions/spelling/allow/allow.txt

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