Allow controlling text of documentation link.

[lrhn]

If you write [Foo.bar], you link to Foo.bar and the link text is Foo.bar.

There is no way to link to Foo.bar with a text of, fx, Foo.current.bar. Or anything else.

Currently if you write, as I did: [`Platform.current.nativePlatform`][Platform.nativePlatform],
the link does link to the documentation for Platform.nativePlatform, but the text is still Platform.nativePlatform too.

I don't know where the link text went. It's not in the output at all.
I'm pretty sure this is actually a bug, because DartDoc tries to not intercept Markdown reference-style links ([text][reference]) like:

The [banana][Bananas] is fine.

[Bananas]: http://example.com/bananas "example of bananas"

so it should have ignored the [Platform.nativePlatform].

It would be nice if it had actually worked. When DartDoc sees a markdown reference-style link, and it cannot find the target for the link (no Platform.nativePlatform link target defined in the document) then DartDoc should take over and try to interpret it as a doc element link.
It would let you make nice links to any documentable element, with full control over the link text.

Then a normal dartdoc link like [Platform.nativePlatform] is just short for [`Platform.nativePlatform`][Platform.nativePlatform].

[lrhn]

A little more context.

For an example DartDoc like:

/// ### Normal reference link
/// Has link reference definition:
/// ```md
/// [Example]: https://example.com "original title text"
/// ```
///
/// * `[Example]` shortcut reference link: [Example].
/// * `[Example][]` collapsed reference link: [Example][].
/// * `[My text][Example]` full reference link: [My Text][Example].
///
/// [Example]: https://example.com/ "original title text"
///
/// ### DartDoc reference link
///
/// Reference link text denotes Dart declaration in scope.
///
/// * `[OtherClass]` shortcut reference link: [OtherClass].
/// * `[OtherClass][]` collapsed reference link: [OtherClass][].
/// * `[My text][OtherClass]` full reference link: [My Text][OtherClass].

the links all work, but [My Text][OtherClass] is rendered exactly the same as
[OtherClass] and [OtherClass][]:

Screenshot