[Go] Add Super property for Base*Visitor struct

[terwey]

Add a super property to the Base*Visitor generated code allowing to only override the required Visitors vs implementing each one in its entirety.

Example code:

	type fooVisitor struct {
		*parser.BaseFooVisitor
	}

	is := antlr.NewInputStream("4+6")
	lexer := parser.NewAnalyzerLexer(is)
	stream := antlr.NewCommonTokenStream(lexer, antlr.TokenDefaultChannel)
	p := parser.NewAnalyzerParser(stream)

	fv := &fooVisitor{}
	fv.BaseFooVisitor = &parser.BaseFooVisitor{}
	
	// this is new
	fv.BaseAnalyzerVisitor.SetSuper(fooVisitor)
	fv.Visit(p.Prog())

[mitar]

This is pretty interesting approach.

[parrt]

@terwey is this superseded by https://github.com/antlr/antlr4/pull/3299 ?