Expose "aliased_html" in doc JSON, make "aliased" field nullable

[oprypin]

I split out this change from #10109 because it's not strictly related.

Same as there, I need this for my external doc generator, to detect where Crystal decides to linkify types and such.

Example output:

"alias": true,
"aliased": "Slice(UInt8)",
"aliased_html": "<a href=\"Slice.html\">Slice</a>(<a href=\"UInt8.html\">UInt8</a>)",

"alias": false,
"aliased": null,
"aliased_html": null,

Before:

"alias": true,
"aliased": "Slice(UInt8)",

"alias": false,
"aliased": "",

[oprypin]

Ping :S
This is a really small one

[oprypin]

Thank you!