Compiletest: Simplify {Html,Json}DocCk directive handling

[fmease]

So much more maintainable and extensible.

r? @jieyouxu as discussed

[rustbot]

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[fmease]

In a roundabout way this used to check that the separator between the directive and its args was a space because the capture group args used to immediately follow the directive name.

[fmease]

I basically had to extend the grammar of {Html,Json}DocCk directives to admit : as a legal separator besides whitespace.

Otherwise it would've either been included in the args if I hadn't relaxed cmd's grammar (so //@ has: ... would've meant [has, : ...]) which would've lead to bad diagnostics down the line like "unexpected argument" (because : ... would've been further split into [:, ...] by shlex) or it would've been accepted by compiletest but rejected ignored by HtmlDocCk (remember, we can't reject any unknown or malformed directive in HtmlDocCk since it no longer knows if it's a compiletest directive or not).

Disregarding the technical reasons, {Html,Json}DocCk directives should be extensions to regular compiletest directives, so it only makes sense that : is allowed as a separator.