[Go] Fix for #4342--EmitToken(token) in Go target needs to be exported

[kaby76]

This is a fix for #4342.

This change is required for porting https://github.com/antlr/grammars-v4/tree/master/python/python3 to the Go target.

This change adds EmitToken(token) to the type interface for lexers. It's required because it's available--and used!!--in all the other targets. In addition, the call to EmitToken(token) must be "virtualized", which is accomplished using b.Virt.EmitToken(token). The Virt field is set prior to calling the parse in the driver code.

This change is a fix to what is just a terrible, terrible hack. A v-table call should have never been implemented this way!

var lexer = parser.NewPython3Lexer(str)
lexer.BaseLexer.Virt = lexer
var tokens = antlr.NewCommonTokenStream(lexer, 0)
var parser = parser.NewPython3Parser(tokens)
var tree = parser.File_input()

[parrt]

@jimidle ?