feat: add count and query APIs for events and text runs at the Artboa…

…rd level, and expose in WASM

Will add some tests, but want to get validation before I go much further that this is the approach we want to take to expose Events and Text Runs for a given artboard at the `rive::Artboard` level, similar to the APIs for getting the linear animation/state machine from an `Artboard`. Adding to `ArtboardImporter` seemed most appropriate but not sure there's any caveats or better ways to plumb through adding the events/runs. Maybe @luigi-rosso @philter ?

Decided on exposing `textValueRunCount()`/`textValueRunAt()` and `eventCount()`/`eventAt()` APIs on the Artboard at the cpp level to follow existing patterns. This should make it simple for other runtimes to adopt too. Another alternative is just doing an empty `find<rive::TextValueRun>()`/`find<rive::Event>()` call at each runtime level which looks like it returns a list of all the objects.

TODO:
- [x] Add tests
- [x] Check WASM size before/after new bindings

Diffs=
22077beda feat: add count and query APIs for events and text runs at the Artboard level, and expose in WASM (#6043)

.rive_head

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-f95f54140657f8b8726392a3d0b4036bb4ad70fb
+22077bedae4c277e5e8a81c732167ab5d559917c

.rive_renderer

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-cd2acf7763d53681991b732b7de4737cf88731d8
+9104efd47835d559c1f685129e8c1224afae39f3