Add settings UI page

[franknoirot]

Resolves tauri-apps/tauri#160. Does the following:

• Adds a new AppHeader component
• Adds react-router for navigation
• Adds a new /settings route accessible through the Settings button in the AppHeader
• Adds Tauri-only UI to either manually enter a new default directory or use the file picker to select one
  • Note that Tauri doesn't yet support the New Folder button on MacOS but we can either help them with that issue or wait for them to finish it.
• Adds UI to edit a new "default project name" setting that does nothing at the moment.
• Adds react-hot-toast to give a success message on save, and support future notification UI.

Demo

Screenshare.-.2023-07-12.8_43_37.PM.mp4

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Name	Status	Preview	Updated (UTC)
untitled-lang	✅ Ready (Inspect)	Visit Preview	Jul 13, 2023 4:36am

[Irev-Dev]

Looks great.
It would be good to get tests passing again before we merge.