ci: add test with older TypeScript version

.github/workflows/ci.yml

@@ -17,6 +17,7 @@ jobs:
     strategy:
       matrix:
         node-version:
+          - 14
           - 20
 
     steps:
@@ -31,5 +32,11 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      # the "override" keyword was added in typescript@4.5.0
+      # else, users can go down to typescript@3.8.x ("import type")
+      - name: Install TypeScript 4.5
+        run: npm i typescript@4.5
+        if: ${{ matrix.node-version == '14' }}
+
       - name: Run tests
         run: npm test