Prototype a continuous integration workflow

This workflow is too spendthrift for deployment: every job sets up Rust,
and the `build` job also installs Trunk, which takes even more
resources. The workflow serves well, however, as a proof of concept.
Introducing a syntax error into `src/main.rs` causes the `build` and
`test` jobs to fail, but allows `run-examples` to succeed. Similarly,
introducing a syntax error into `examples/point-on-sphere.rs` causes
`run-examples` to fail, but allows the other jobs to succeed.

.forgejo/setup-rust/action.yaml

@@ -0,0 +1,10 @@
+# set up the Rust toolchain. based on David Tolnay's more general
+# `rust-toolchain` action for GitHub
+#
+#   https://github.com/dtolnay/rust-toolchain
+#
+runs:
+  using: "composite"
+  steps:
+    - run: curl --proto '=https' --tlsv1.2 --retry 10 --retry-connrefused --location --silent --show-error --fail 'https://sh.rustup.rs' | sh -s -- -y --profile minimal
+    - run: echo $HOME/.cargo/bin >> $GITHUB_PATH