Add content assist for @NamedVariant, et al.

[eric-milles]

Consider the following:

import groovy.transform.*

class Color {
  int r, g, b
}

@NamedVariant def m(Color c) { ... }

m(|)
  ^ Ctrl-Space gives proposals for "r: __", "g: __", "b: __" and "meth(Map namedArgs)"

And also:

import groovy.transform.*

@NamedVariant def m(@NamedParam('dob') Date date) { ... }

m(|)
  ^ proposals for "dob: __" but not "date: __"