Fix goto definition for field names.

[tritao]

Changes Tree-Sitter field_name to contain a reference_expression.

On top of #134

[spytheman]

Why is there a need for it?
Can you show a small V code sample, that is now handled, but was not before?

[tritao]

Why is there a need for it? Can you show a small V code sample, that is now handled, but was not before?

fn main() {
	mut cmd := cli.Command{
		name:        metadata.manifest.name
		version:     metadata.full_version
		description: metadata.manifest.description
		execute:     run
		posix_mode:  true
	}

Sure. Any time you try to do goto-definition on a field name, like name: above. It didn't work before because all goto definition code is built with expectation of reference_expression. I tried it fixing some other way but this turned out to be a much simpler fix.