revert: "ci: don't run on push"

This reverts commit d1369fc.

The reasoning is provided by @justinmk in
#122 (comment):

It's good to save CI time, but for low-activity repos I think we should
err on the side of explicitness.

Reasons in favor of CI on push:
  - to have a baseline, so one can see the history of status checks by looking at master (including squash-merged PRs)
  - for any (rare) commits that skip the PR process

Without this, we don't have a clear concept of "current status", except
whatever the last PR was (which also assumes a linear history).

.github/workflows/main.yml

@@ -1,14 +1,21 @@
 name: Run tree-sitter tests
 
 on:
+  push:
+    branches: [ master ]
   pull_request:
+    branches: [ master ]
 
 jobs:
   test:
     name: Tree-sitter tests
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+
       - uses: actions/setup-node@v4
-      - run: npm install
-      - run: npm test
+      - name: Install Dependencies
+        run: npm install
+
+      - name: Run tests
+        run: npm test