GDScript: Use "is" keyword for type checking.

Replaces the `extends` keyword with `is` in the context of testing for type compatibility.
`extends` is still used for declaring class inheritance.

Example:

```gdscript
extends Node2D

func _input(ev):
	if ev is InputEventKey:
		print("yay, key event")
```

modules/gdscript/gd_compiler.cpp

@@ -1014,7 +1014,7 @@ int GDCompiler::_parse_expression(CodeGen &codegen, const GDParser::Node *p_expr
 					}
 
 				} break;
-				case GDParser::OperatorNode::OP_EXTENDS: {
+				case GDParser::OperatorNode::OP_IS: {
 
 					ERR_FAIL_COND_V(on->arguments.size() != 2, false);
 

modules/gdscript/gd_function.cpp

@@ -358,12 +358,12 @@ Variant GDFunction::call(GDInstance *p_instance, const Variant **p_args, int p_a
 
 				if (a->get_type() != Variant::OBJECT || a->operator Object *() == NULL) {
 
-					err_text = "Left operand of 'extends' is not an instance of anything.";
+					err_text = "Left operand of 'is' is not an instance of anything.";
 					break;
 				}
 				if (b->get_type() != Variant::OBJECT || b->operator Object *() == NULL) {
 
-					err_text = "Right operand of 'extends' is not a class.";
+					err_text = "Right operand of 'is' is not a class.";
 					break;
 				}
 #endif
@@ -401,7 +401,7 @@ Variant GDFunction::call(GDInstance *p_instance, const Variant **p_args, int p_a
 
 					if (!nc) {
 
-						err_text = "Right operand of 'extends' is not a class (type: '" + obj_B->get_class() + "').";
+						err_text = "Right operand of 'is' is not a class (type: '" + obj_B->get_class() + "').";
 						break;
 					}
 

modules/gdscript/gd_parser.cpp

@@ -1077,7 +1077,7 @@ GDParser::Node *GDParser::_parse_expression(Node *p_parent, bool p_static, bool
 			case GDTokenizer::TK_OP_BIT_AND: op = OperatorNode::OP_BIT_AND; break;
 			case GDTokenizer::TK_OP_BIT_OR: op = OperatorNode::OP_BIT_OR; break;
 			case GDTokenizer::TK_OP_BIT_XOR: op = OperatorNode::OP_BIT_XOR; break;
-			case GDTokenizer::TK_PR_EXTENDS: op = OperatorNode::OP_EXTENDS; break;
+			case GDTokenizer::TK_PR_IS: op = OperatorNode::OP_IS; break;
 			case GDTokenizer::TK_CF_IF: op = OperatorNode::OP_TERNARY_IF; break;
 			case GDTokenizer::TK_CF_ELSE: op = OperatorNode::OP_TERNARY_ELSE; break;
 			default: valid = false; break;
@@ -1117,7 +1117,7 @@ GDParser::Node *GDParser::_parse_expression(Node *p_parent, bool p_static, bool
 
 			switch (expression[i].op) {
 
-				case OperatorNode::OP_EXTENDS:
+				case OperatorNode::OP_IS:
 					priority = -1;
 					break; //before anything
 
@@ -1420,7 +1420,7 @@ GDParser::Node *GDParser::_reduce_expression(Node *p_node, bool p_to_const) {
 				}
 			}
 
-			if (op->op == OperatorNode::OP_EXTENDS) {
+			if (op->op == OperatorNode::OP_IS) {
 				//nothing much
 				return op;
 			}

modules/gdscript/gd_parser.h

@@ -216,7 +216,7 @@ public:
 			OP_CALL,
 			OP_PARENT_CALL,
 			OP_YIELD,
-			OP_EXTENDS,
+			OP_IS,
 			//indexing operator
 			OP_INDEX,
 			OP_INDEX_NAMED,

modules/gdscript/gd_script.cpp

@@ -1812,6 +1812,7 @@ void GDScriptLanguage::get_reserved_words(List<String> *p_words) const {
 		"breakpoint",
 		"class",
 		"extends",
+		"is",
 		"func",
 		"preload",
 		"setget",

modules/gdscript/gd_tokenizer.cpp

@@ -90,6 +90,7 @@ const char *GDTokenizer::token_names[TK_MAX] = {
 	"func",
 	"class",
 	"extends",
+	"is",
 	"onready",
 	"tool",
 	"static",
@@ -864,6 +865,7 @@ void GDTokenizerText::_advance() {
 								{ TK_PR_FUNCTION, "func" },
 								{ TK_PR_CLASS, "class" },
 								{ TK_PR_EXTENDS, "extends" },
+								{ TK_PR_IS, "is" },
 								{ TK_PR_ONREADY, "onready" },
 								{ TK_PR_TOOL, "tool" },
 								{ TK_PR_STATIC, "static" },

modules/gdscript/gd_tokenizer.h

@@ -96,6 +96,7 @@ public:
 		TK_PR_FUNCTION,
 		TK_PR_CLASS,
 		TK_PR_EXTENDS,
+		TK_PR_IS,
 		TK_PR_ONREADY,
 		TK_PR_TOOL,
 		TK_PR_STATIC,