ci: run build on: PR, pushes to main and tags

- Running on PR makes it easier to debug the build process.
- Running on main makes it easier to test the code in main.
- Running on tags is good for releases. Later we can make the CI publish
  the vsix as a release when a tag is created.

Author: agateau-gg

.github/workflows/build.yaml

@@ -1,8 +1,12 @@
 name: vsixbuilder
 
 on:
+  pull_request:
   push:
-    branches: [main]
+    branches:
+      - "main"
+    tags:
+      - "v*"
 
 permissions:
   contents: read