chore: ask in the PR template whether a change is breaking (#3782)

The template now asks every PR whether it breaks existing apps,
and if it does, asks for both markers. It mentions the commit
subject as well as the PR title because a one-commit PR lands
on main with its commit's subject.

Author: armando-navarro

PULL_REQUEST_TEMPLATE.md

@@ -25,6 +25,7 @@ before sending PRs. We cannot accept code without this.
    - Issue number for this PR: #nnn (required)
    - Docs included?: (yes/no; required for all API/functional changes) 
    - Test units included?: (yes/no; required) 
+   - Breaking change?: (yes/no, required. If yes, add a `BREAKING CHANGE:` footer to your commit message that says what an upgrading app must change, and put `!` before the colon in the PR title and the commit subject, like `fix(auth)!: ...`)
    - In a clean directory, `yarn install`, `yarn test` run successfully? (yes/no; required)
 
 ### Description