feat(plugins): notifications targeted at one project (API 1.5) (#2375)

- Add ui.notify-project: it shows only in that project's window (the most recently focused one if there are several) and works without a calling window
- Button commands run in the window showing the notification; a closed project returns not-found, with nothing shown and no fallback to another window
- Bump the plugin API to 1.5.0 (wit/, SDK copy, ENGINE_VERSION, API_VERSION) and rebuild all components
- Add Project::notify and its mock to the SDK; update plugins/README.md

Co-authored-by: LU XIANGRUI <[email protected]>

wit/plugin.wit

@@ -8,7 +8,7 @@
 /// `fs` and `workspace`, and everything else goes through the imports below,
 /// gated by the capabilities granted in `plugin.toml` (a missing one returns
 /// `error::denied`).
-package openidea:plugin@1.4.0;
+package openidea:plugin@1.5.0;
 
 world plugin {
     import log;