[megathread] Snippets

[DHowett]

maintained by @zadjii-msft

### 1.22 Tasks
- [x] #1595
- [x] #16063
- [x] #15664
- [x] #17329
- [x] #17330
- [x] #17388  Add support for local snippets (`.wt.json`, `.wtconfig`)
- [x] #16513
- [x] https://github.com/microsoft/terminal/issues/12857
- [x] https://github.com/microsoft/terminal/issues/17366

### 1.23
- [ ] #17805
- [ ] Search results existing inside nested snippets should expand them

WIP Branches:

• Local snippets:
  • dev/migrie/fhl/local-tasks-2024 was the branch from the last day of Spring FHL 2024, but that branched off the md-pane prototype.
  • ~dev/migrie/f/just-local-tasks~ is me trying to isolate that commit
  • dev/migrie/f/just-local-snippets I started working on, till I realized I'd need to work on top of #17281
  • dev/migrie/f/local-snippets-on-action-refactor
    • I think https://github.com/microsoft/terminal/compare/dev/pabhoj/action_new_tab_menu...dev/migrie/f/local-snippets-on-action-refactor1 is ready, but turns out that action_new_tab_menu didn't have main merged in, so the history is confusing now
    • https://github.com/microsoft/terminal/compare/dev/pabhoj/action_new_tab_menu...dev/migrie/f/local-snippets-cleaner has a cleaner delta, I think

Related

• #15845
• #17344