(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

[rdhar]

@FnhNielsen Thank you for raising this PR -- I will be following up on this feature in the upcoming release!

I'm curious if you noticed the presence of invisible comments like <!-- placeholder-1 -->? Perhaps it might be an idea to support passing markdown into any/all of these placeholders, while trying to find a balance between flexibility and (excessive) customizability.

[rdhar]

@FnhNielsen Thanks to building on top of your foundation, I've extended it from a singular prefix/title to a number of positional inputs. This gives the user more flexibility to choose exactly where to interpolate their markdown content. For example, you could pass:

• comment-pos-1: "### ${{ matrix.environment }}" for a dynamic header title based on the environment; or
• comment-pos-2: ${{ steps.tflint-comment.outputs.result }} to insert markdown-formatted output from a prior step of the workflow (see TFLint example).