Include ESM .mjs file in size comparisons

Author: markerikson

.github/workflows/size.yaml

@@ -14,4 +14,4 @@ jobs:
       - uses: preactjs/compressed-size-action@v1
         with:
           repo-token: '${{ secrets.GITHUB_TOKEN }}'
-          pattern: './{dist,es,lib}/*.js'
+          pattern: './{dist,es,lib}/*.{js,mjs}'