Meta: adopt the WHATWG PR template (#124)

PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+<!--
+Thank you for contributing to The Storage Access API! Please describe the change you are making and complete the checklist below if your change is not editorial.
+-->
+
+- [ ] At least two implementers are interested (and none opposed):
+   * …
+   * …
+- [ ] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
+   * …
+- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
+   * Chromium: …
+   * Gecko: …
+   * WebKit: …
+
+(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)