Add some sort of data persistence to the cover letter view

[SuzGupta]

We need to store data somewhere so we can demonstrate pausing this and resume this around an active Writing Tools session so Writing Tools does not mess up the stored text.

Pasting in from discussion with @drmarkpowell:

• Store each cover letter and the skills in SwiftData
• Make some SwiftData classes and start saving them
• Make sure we can use a binding to connect up one of the letter text String parameters (Letter.body?) to the UIKit-hosted text view and edit it successfully

[SuzGupta]

Add models for

• address block
• salutation
• closing
• letter
• skill

[SuzGupta]

Adding further thoughts from @drmarkpowell as we discuss an easy yet meaningful way to demonstrate the tools pause and resume:

Maybe we can just write changes to a file in documents, or UserDefaults?
That’s all Josh did for App Intents…put all his “database” into a local data file
Maybe we could have a simple “file backup” handler that version controls your edits as you write, and just disable/enable that.
Then it’s completely under your control
You can just define a struct that’s Codable and read/write it at the proper time whenever you see fit, and “pause” it for Writing Tools changes

Link to Josh's repo to see what we can glean: https://github.com/kodecocodes/m3-sai-materials