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]>

plugins/official/string-tools/dist/plugin.wasm.sources

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-c2feb6a4cd0e9908f078a3d0074732cfce653cc57cce9d1907838598515048ee
+65fa71854546a7b19f7afe86642bc1e2c4eee9a7a08a71f87d307c68c95410d0