common-errors.go: exported all error functions.

builtin-string.go: renamed all functions from somthingStr() to strSomething()

t_builtin-base_test.go

@@ -1,7 +1,7 @@
 // Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.
 
-// t_func-base_test.go
+// t_builtin-base_test.go
 package expr
 
 import (
@@ -10,7 +10,7 @@ import (
 )
 
 func TestFuncBase(t *testing.T) {
-	section := "Func-Base"
+	section := "Builtin-Base"
 
 	inputs := []inputType{
 		/*   1 */ {`isNil(nil)`, true, nil},