lexers/cue: support CUE attributes

[myitcv]

Currently the following CUE results in the chroma lexer producing an error token for the '@':

value: string @go(Value)

This code is, however, valid CUE. '@go' is an attributes.

This change adds lexer support for attributes in CUE.

[myitcv]

Gentle nudge

[alecthomas]

Apologies!