Add CI (fmt, clippy, test, wasm web build)

#1 in Riparion/riparion-editor — merged 2026-06-02

Adds a GitHub Actions workflow now that the crate lives in its own repo (it's no longer covered by riparion-cms's CI).

Two jobs, mirroring the sibling dx-dnd setup but adapted for a single-crate repo:

• host — cargo fmt --check, cargo clippy --all-targets -D warnings, and the round-trip split/join unit tests.
• wasm32 — cargo clippy --features web on wasm32-unknown-unknown (the browser build: web-sys caret/selection + dx-dnd's web drag).

All four commands verified passing standalone before pushing. This PR's own run exercises the new workflow.

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