feat(plugins): leave room in plugin API 1.0 for what will grow

Released 1.x types never change, so the sets and records that will have
to grow are opened up before the API is frozen:

- Event kinds and action sources are strings; `event`, view `body`,
  `view-event` and `field-value` get an `other(extension)` case (a kind and
  JSON data) that carries later cases to older plugins and IDEs. An IDE
  refuses an event kind it doesn't know, says a view body needs a newer
  IDE, and leaves out unknown form fields.
- Completion items can be snippets, match on `filter-text` and bring
  `additional-edits` (imports). Code actions can carry `data` instead of
  edits; the new `resolve-code-action` export computes them when the user
  picks the action, under the command budget, through the existing resolve
  path.
- Diagnostics name their `source`, carry `tags` (`unnecessary`,
  `deprecated`) and `related` locations.
- The SDK's enums are `#[non_exhaustive]` with `Other` variants;
  `Diagnostic::new`, `Providers::resolve_code_action`.
- README: 1.x calls stay synchronous and pass documents whole; async and
  incremental sync come later behind the host adapter or a new interface.

Official plugins and fixtures rebuilt.

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

@@ -1 +1 @@
-09cc92e8e731f8f59f3857a71f9a30cc3d9e04a1c32092920f573b0f26592855
+04844bfbfbcb1770f4f4742658944c592922b037b2387e9340de58ef0d9baacc