refactored data-types to reduce plugin sizes

import-utils.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 
 	"git.portale-stac.it/go-pkg/expr/kern"
+	"git.portale-stac.it/go-pkg/expr/types/str"
 	"git.portale-stac.it/go-pkg/expr/util"
 )
 
@@ -22,7 +23,7 @@ const (
 )
 
 func checkStringParamExpected(funcName string, paramValue any, paramPos int) (err error) {
-	if !(kern.IsString(paramValue) /*|| isList(paramValue)*/) {
+	if !(str.IsString(paramValue) /*|| isList(paramValue)*/) {
 		err = fmt.Errorf("%s(): param nr %d has wrong type %s, string expected", funcName, paramPos+1, kern.TypeName(paramValue))
 	}
 	return