Allow references to scoped labels by title

What does this MR do and why?

See: #352700 (closed)

When the title is simple in all parts, then allow referencing it by title.

For example: ~foo:bar, ~foo::bar, and ~foo::bar::baz are all simple in all parts, and are accepted by the parser.

A label such as ~"foo::two parts" or ~"foo bar::one" requires quoting.

How to set up and validate locally

Use the following quick-action on this MR:

/label ~workflow::blocked

Notice that it does not succeed - and the label is not even parsed.

Use a similar label command using the code from this branch, and see that it works.

MR acceptance checklist

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