Highlight indented method signatures in docstrings by default

[christopher-dG]

Here's my attempt at closing #978.

When highlightsig is not set to false in makedocs, leading, unlabeled code blocks in docstrings (i.e. the usual indented signature) are labeled as Julia.

Open to any bikeshedding 😅

[mortenpi]

Implementation LGTM! Could use a (1) entry in the makedocs docstring, (2) a CHANGELOG entry, and (3) perhaps a few simple tests for highlightsig!, e.g. that the following ones get transformed correctly:

    foo
---
    bar

```xyz
foo
```

[christopher-dG]

Ok, I have done those things. I put the tests in runtests.jl since I didn't find any other file relevant to Expanders. And any idea why the changelog doesn't seem to want to render the link to this PR? I may be somewhat overtired but the raw text looks identical to the other links...

[mortenpi]

I put the tests in runtests.jl since I didn't find any other file relevant to Expanders.

That's fine.

And any idea why the changelog doesn't seem to want to render the link to this PR?

It needs the URL down at the bottom of the file.

[christopher-dG]

Oh cool, thanks. I'll take care of that first thing in the morning.

[christopher-dG]

Ok, fixed now.

[mortenpi]

Lovely, thank you!