Add github step summary

[lineargraph]

Adds the reuse tool output as markdown to the summary page of the action

[mxmehl]

Thank you for your contribution.

With a simple usage of the action, this works well, but if you look at the summary of the checks against this pull request, you see that e.g. running the action multiple times or with flags like --json may create odd results: https://github.com/fsfe/reuse-action/actions/runs/5870075466

Therefore, my suggestion would be to make the summary configurable, enabled by default. I'm not sure what's the best way to implement that. Perhaps environment variables? I wouldn't choose args as these should be the available arguments to reuse.

[lineargraph]

I implemented the option to disable/change the output using a with: argument. This also enabled prettily formatted json output (although that one tends to get verbose quickly).

[mxmehl]

I implemented the option to disable/change the output using a with: argument. This also enabled prettily formatted json output (although that one tends to get verbose quickly).

Thank you! Looks good on first sight but I would like to test and understand a bit more in detail. Not sure whether I can manage this before my vacation, but it's on my todo list.

[GooRoo]

Hey guys, what's with this PR? Is it going to be merged?

[mbtools]

there's a default. why is it mandatory?