first plugin support.

Module organization requires a better structure to decouple definitions and implementations

symbol.go

@@ -101,6 +101,7 @@ const (
 	SymKwBut
 	SymKwFunc
 	SymKwBuiltin
+	SymKwPlugin
 	SymKwIn
 	SymKwInclude
 	SymKwNil
@@ -113,6 +114,7 @@ func init() {
 	keywords = map[string]Symbol{
 		"AND":     SymKwAnd,
 		"BUILTIN": SymKwBuiltin,
+		"PLUGIN":  SymKwPlugin,
 		"BUT":     SymKwBut,
 		"FUNC":    SymKwFunc,
 		"IN":      SymKwIn,