PR body: use html delimiters to allow appending or prepending to content

[rarkins]

Describe the proposed change(s).

We should start adding <!--renovate-pr-body-start--> before our content and <!--renovate-pr-body-end--> after it, and ignoring any content before or after that. If either tag is missing then we assume that the start or end of the content is the start or end of the PR body itself.

We should not proactively edit every existing PR and cause a lot of edits, but if we are already editing an existing PR then we should add these delimiters. If editing a PR then we should retain any content before or after.

For platforms which don't support hidden html comments, we should strip this content out, which means we don't support PR body editing on those platforms.