ci: add macos-latest to self-test matrix and document cross-platform runner support

Back the macOS support claim by running the self-test workflow on
macos-latest in addition to ubuntu-latest and windows-latest, and add a
README note that the action runs on Linux, macOS, and Windows runners.

Author: shenxianpeng

.github/workflows/commit-check.yml

@@ -10,7 +10,7 @@ jobs:
   commit-check:
     strategy:
       matrix:
-        os: [ubuntu-latest, windows-latest]
+        os: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.os }}
     permissions:
       contents: read