Only replace ampersands if they don't belong to another HTML entity

[stroborobo]

Inline ampersands were previously replaced by &, even if they already belonged to another entity.

For example this:

Type: **'T -> 'U**

Became this:

<p>Type: <strong>'T -&amp;gt; 'U</strong></p>

But it should have been this:

<p>Type: <strong>'T -&gt; 'U</strong></p>

I'm fairly new to F#, so I hope this is PR is ok.

[wallymathieu]

Looks like an improvement. I like that there are added tests 👍

[wallymathieu]

What do you think @matthid ?

[matthid]

I'm not sure, what does the markdown spec say to this?

[stroborobo]

@matthid

All valid HTML entity references and numeric character references, except those occuring in code blocks and code spans, are recognized as such and treated as equivalent to the corresponding Unicode characters. Conforming CommonMark parsers need not store information about whether a particular character was represented in the source using a Unicode character or an entity reference.

https://spec.commonmark.org/0.28/#entity-and-numeric-character-references

[stroborobo]

Nice, thanks for merging, guys!

[wallymathieu]

Thanks for the contribution @stroborobo ! 👍