Fix type inferencing, syntax highlighting, code navigation and content assist for static reference to extension method

[eric-milles]

#878 fixed String::length references. Groovy 3 also supports pointers/references to extension methods.

def f = String.&length
println f('123')

def g = String.&size // DGM
println g('123')

def zzz = Object.&sleep // DGSM
zzz(123)

"size" and "sleep" are shown as unknown (underlined).

[eric-milles]