Add GitHub Actions CI workflow

Build matrix: ubuntu-latest + macos-latest x OCaml 5.0 / 5.4. Plus an
opam lint job. Modelled after the workflow shipped by html_of_wiki.

Windows is intentionally excluded: the build relies on POSIX make and
the pcre/lwt dependencies are not exercised on the win32 runner here.

Author: balat

.github/workflows/workflow.yml

@@ -0,0 +1,48 @@
+name: Main workflow
+
+on:
+  pull_request:
+  push:
+  schedule:
+    # Prime the caches every Monday
+    - cron: 0 1 * * MON
+
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - ubuntu-latest
+          - macos-latest
+        ocaml-compiler:
+          - "5.0"
+          - "5.4"
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout tree
+        uses: actions/checkout@v4
+
+      - name: Set-up OCaml
+        uses: ocaml/setup-ocaml@v3
+        with:
+          ocaml-compiler: ${{ matrix.ocaml-compiler }}
+
+      - run: opam install . --deps-only
+
+      - run: opam exec -- make all
+
+  lint-opam:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout tree
+        uses: actions/checkout@v4
+
+      - name: Set-up OCaml
+        uses: ocaml/setup-ocaml@v3
+        with:
+          ocaml-compiler: "5.4"
+
+      - uses: ocaml/setup-ocaml/lint-opam@v3

CHANGES

@@ -13,3 +13,5 @@
   * Add LICENSE / LICENSE.GPL-2 / LICENSE.QPL-1.0 documenting the
     per-component licensing (latex_of_wiki under GPL-2-or-later,
     odoc_wiki under QPL-1.0 as a derivative of the ocamldoc generators).
+  * Add a GitHub Actions workflow building on Ubuntu and macOS against
+    OCaml 5.0 and 5.4, plus an opam lint job.