Suppor for inline ref-syntax (pro-only)

[josteink]

As seen in the documentation, the keyword ref has several forms.

Single-line statement identified by a colon:

ref over Server: LISTEN

Or as multi-line statements in the following format:

ref over K, K
Kim makes a sandwich
end ref

It also supports a form on inline syntax with other WSD-syntax:

Alice->ref over Bob, Mary: input
   refer to other diagram
end ref -->Alice: output

Currently wsd-mode only supports the first two syntaxes, and not the third.

Not supporting this syntax means that we're not 100% feature complete syntax-wise, but supporting this specific syntax is currently not a priority.

This issue is first and foremost meant to document to self (and others) that this shortcoming exists.

CC: @dlasko