Go: fix highlighting of non-reserved keywords (atoms)

https://github.com/codemirror/codemirror5/blob/78555dd4ac9bc691f081eec8266a01d3fbcc0d4e/mode/go/go.js#L30

Words in this list are highlighted as special keywords regardless of context. For example, it is valid in Go to create a variable called "new". It should only receive special highlighting when used as the builtin function new(expr), and should be treated as a variable otherwise.