[megathread] Snippets

_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

### Future

- [ ] Snippets pane needs a button to add a snippet
* with options for saving to the settings.json,
* or to the `.wt.json` currently found. (like, save it to the most local wt json)
- [ ] Command decorator (driven by marks #11000) should include a "save as snippet" command
- [ ] Snippets needs to not be preview-only

### WIP Branches:

* Local snippets:
* [`dev/migrie/fhl/local-tasks-2024`](https://github.com/microsoft/terminal/tree/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`](https://github.com/microsoft/terminal/tree/dev/migrie/f/just-local-tasks)~ is me trying to isolate that commit
* [`dev/migrie/f/just-local-snippets`](https://github.com/microsoft/terminal/tree/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`](https://github.com/microsoft/terminal/tree/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

Start by reviewing the unchecked 1.23 and Future entries, then read related issues #15845 and #17344 to identify a single scoped task. The issue names no files or tests, and completion criteria are not specified here, so scope and acceptance criteria need confirmation before starting.

Written by the indexing model from the issue text.