feat: Continuous integration via Forgejo Actions/runners (#75)

Adds a continuous integration workflow to the repository, using the [Forgejo Actions](https://forgejo.org/docs/next/user/actions/) framework.

Concurrently, Aaron added a [wiki page](https://code.studioinfinity.org/glen/dyna3/wiki/Continuous-integration) to document the continuous integration system. In particular, this page explains how to [run continuous integration checks on a development machine](wiki/Continuous-integration#execution), either directly or in a container.

Co-authored-by: Aaron Fenyes <aaron.fenyes@fareycircles.ooo>
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: StudioInfinity/dyna3#75
Co-authored-by: Vectornaut <vectornaut@nobody@nowhere.net>
Co-committed-by: Vectornaut <vectornaut@nobody@nowhere.net>

.forgejo/setup-trunk/action.yaml

@@ -0,0 +1,22 @@
+# set up the Trunk web build system
+#
+#   https://trunkrs.dev
+#
+# the `curl` call is based on David Tolnay's `rust-toolchain` action
+#
+#   https://github.com/dtolnay/rust-toolchain
+#
+runs:
+  using: "composite"
+  steps:
+    - run: rustup target add wasm32-unknown-unknown
+    
+    # install the Trunk binary to `ci-bin` within the workspace directory, which
+    # is determined by the `github.workspace` label and reflected in the
+    # `GITHUB_WORKSPACE` environment variable. then, make the `trunk` command
+    # available by placing the fully qualified path to `ci-bin` on the
+    # workflow's search path
+    - run: mkdir -p ci-bin
+    - run: curl --output - --proto '=https' --tlsv1.2 --retry 10 --retry-connrefused --location --silent --show-error --fail 'https://github.com/trunk-rs/trunk/releases/download/v0.21.12/trunk-x86_64-unknown-linux-gnu.tar.gz' | tar --gunzip --extract --file -
+      working-directory: ci-bin
+    - run: echo "${{ github.workspace }}/ci-bin" >> $GITHUB_PATH