Suggest mode 1/7: editor intent (edit/suggest/view) and experiment gate

[adamsilverstein]

What's in this PR

Adds the session-scoped editor intent state (private setEditorIntent
action / getEditorIntent selector), the Edit / Suggest / View switcher in
the editor options menu, Google Docs convention keyboard shortcuts, and a
read-only view intent via isPreviewMode. The Suggestion Mode experiment
flag and the gate helpers (isSuggestionModeEnabled / useCanSuggest) land
here so every later layer gates on the same predicate. No suggestion
capture or storage yet.

---

Suggest mode stack

This rebuilds the manually-stacked Suggest mode work (#73411) as a GitHub Stack of 7 small, independently reviewable PRs. Each is CI-green on its own and builds on the one below it:

1. #80427 - editor intent (edit/suggest/view) + experiment gate
2. #80428 - suggestion storage, REST controller, provider
3. #80429 - block-level capture (attribute + structural)
4. #80430 - inline marker primitive
5. #80431 - inline suggestion operations
6. #80432 - review UI (Apply/Reject sidebar + summary)
7. #80433 - inline live wiring, docs, e2e

The whole feature can be exercised end-to-end via the combined testing branch #78994 (Playground). Behind the "Suggestion Mode" experiment (Gutenberg > Experiments).

[github-actions]

Size Change: +796 B (+0.01%)

Total Size: 7.73 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	500 kB	+796 B (+0.16%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in d7cc4fd.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/29605718528
📝 Reported issues:

• [Flaky Test] should insert media, convert to blocks, and undo in one step #36123 in /test/e2e/specs/editor/blocks/classic.spec.js