(feature): Added support for customization of the PR comment

[FnhNielsen]

This PR introduces a new feature which allows users to customise the the body of the comment left by the action. This feature was requested in #546 and we ended up implementing it ourself and wanted to contribute it.

Fixes #546

Before our contribution

We also validated that this is exactly what it looks like if comment-prefix isn't set.

After our contribution

Here shown with comment-prefix: "# Plan for module <redacted> on `dev`"

[ArkShocer]

I tested your branch with my matrix test job, and I don't see any issues other than it not being a prefix. To me, this would more fall under the category of a title/header.

[birjj]

@ArkShocer Isn't that because you've used a header (# ...) as the prefix? It's the same thing we've done, but I could see others wanting to use something else - entire paragraphs, for example.

[FnhNielsen]

Bump for visibility