Add GitHub Actions workflow for syntax tests

Author: ratijas

.github/workflows/syntax.yml

@@ -0,0 +1,51 @@
+name: Syntax Tests
+
+on:
+  push:
+    paths:
+      - '.github/workflows/syntax.yml'
+      - '**.sublime-syntax'
+      - '**/syntax_test*'
+      - '**.tmPreferences'
+  pull_request:
+    paths:
+      - '.github/workflows/syntax.yml'
+      - '**.sublime-syntax'
+      - '**/syntax_test*'
+      - '**.tmPreferences'
+
+jobs:
+  main:
+    name: Syntax Tests (${{ matrix.build }}, ${{ matrix.packages }})
+    strategy:
+      matrix:
+        include:
+          # latest dev build
+          # This test must pass, the others are optional
+          # due to changes in the Packages repo that affect our tests.
+          - build: latest
+            packages: v4129
+            continue_on_error: false
+          # last dev & Packages `master`
+          - build: latest
+            packages: master
+            continue_on_error: true
+          # latest stable build
+          - build: 4126
+            packages: v4126
+            continue_on_error: true
+          # earliest stable build supported by the current release branch
+          - build: 4107
+            packages: v4107
+            continue_on_error: true
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: SublimeText/syntax-test-action@v2
+        with:
+          build: ${{ matrix.build }}
+          default_packages: ${{ matrix.packages }}
+        # On the step level, this does not cause the entire job to fail
+        # and thus mark PRs as failing when only one of our tests failed.
+        # See also https://github.com/github/feedback/discussions/15452.
+        continue-on-error: ${{ matrix.continue_on_error }}