Refactor GDScript/C# script templates logic to be editor-only

Not a full refactor as it still goes through ScriptLanguage so it's hacky,
but at least it can now compile without this.

modules/gdscript/editor/script_templates/VisualShaderNodeCustom/basic.gd

@@ -0,0 +1,50 @@
+# meta-description: Visual shader's node plugin template
+
+@tool
+class_name VisualShaderNode_CLASS_
+extends _BASE_
+
+
+func _get_name() -> String:
+	return "_CLASS_"
+
+
+func _get_category() -> String:
+	return ""
+
+
+func _get_description() -> String:
+	return ""
+
+
+func _get_return_icon_type() -> int:
+	return PORT_TYPE_SCALAR
+
+
+func _get_input_port_count() -> int:
+	return 0
+
+
+func _get_input_port_name(port: int) -> String:
+	return ""
+
+
+func _get_input_port_type(port: int) -> int:
+	return PORT_TYPE_SCALAR
+
+
+func _get_output_port_count() -> int:
+	return 1
+
+
+func _get_output_port_name(port: int) -> String:
+	return "result"
+
+
+func _get_output_port_type(port: int) -> int:
+	return PORT_TYPE_SCALAR
+
+
+func _get_code(input_vars: Array[String], output_vars: Array[String],
+		mode: int, type: int) -> String:
+	return output_vars[0] + " = 0.0;"