Note missed mutants into GitHub PR annotations

[sourcefrog]

https://github.com/crate-ci/typos/ seems to do this nicely and might be a good example to follow: if there are typos, it flags them like in https://github.com/sourcefrog/cargo-mutants/pull/250/files#annotation_17006893398

This would be complementary to a ready-to-go cargo-mutants action (#141), or maybe it depends on it.

https://github.com/actions/toolkit/blob/master/docs/problem-matchers.md seems to be a way to match regexes in output and create annotations.

But perhaps we can write it out directly:

In the example above, the special ::error::, ::warning::, and ::notice:: syntax is used. The format is:

::level file=path/to/file,line=line_number,col=column_number::message

• level: Can be error, warning, or notice.
• file: The file path relative to the repository root.
• line (optional): The line number in the file.
• col (optional): The column number in the line.
• message: The annotation text.

[siennathesane]

I think writing it out directly is prolly fine! I was looking at how to do this for my stuff