feat: improve support of backtick strings

[aduh95]

Alternate to #94

Only supports code span delimited with a single backtick char and gives up if it finds an edge case.

List of unsupported markdown-compliant edge cases I could think of:

test: string ``code `span` with backticks inside`` not supported
test: string `code span ending with a backslash\\` not supported
test: string containing escaped \` `code span` not supported
`test`: message starting with a backslash not supported

I wasn't sure how to add tests for that, but it works from what I have tested locally.

[rvagg]

looks good, I think.
What you could do for tests is add some trivial commits to this PR and test against the commits that this comprises. See how the tests currently do it.
I'm going to suggest that the next release will be a semver-minor, so you could bake that into your test case with a --start-ref and --end-ref and add commits to this PR with just a change of whitespace in this file, or the README, that have commit messages that form part of the test suite. That's perfectly OK to do here. If we have any awkward breakage during the next release we can sort that out later.
Have a quick go, if it gets too complicated then no big deal I think but it would be nice to see how this works and bake it into the tests.

[rvagg]

looks good I think but the test text fails lint with unnecessary escapes

[rvagg]

sweet

[rvagg]

ok, github didn't like the empty commits, I had to reconstruct that all locally but it should be publisahed now as 2.6.0