Fix parser error with named argument end in macro body

[straight-shoota]

next_macro_token incorrectly recognizes a named argument called end as a macro end delimiter:

def bar(end a)
  a
end

macro foo
  bar(end: 1) # Error: unexpected token: :
end

foo

This patch fixes that. It won't recognize a macro end token if end is immediately followed by :.

This isn't an issue for non-macro tokens because the parser is aware of the context when parsing named arguments.