chore: introduce github actions CI

[jeluard]

Introduce a basic CI workflow.

New environments can be added by modifying the matrix strategy, e.g.:

  - runner: ubuntu-latest
    target: wasm32-unknown-unknown
    optional: true

Note that optional environment failures won't cascade at the workflow level.

By default native ubuntu and macos are tested, and optionally wasm on ubuntu.

In a next step, a better integration with GitHub PR can be considered:

• https://github.com/actions-rust-lang/rustfmt
• https://github.com/actions-rust-lang/audit