GDScript: Fix repeated _ are allowed after decimal point

modules/gdscript/tests/scripts/parser/errors/multiple_number_separators.gd

@@ -1,3 +1,3 @@
 func test():
 	# Number separators may not be placed right next to each other.
-	var __ = 1__23
+	var _num = 1__23

modules/gdscript/tests/scripts/parser/errors/multiple_number_separators.out

@@ -1,2 +1,2 @@
 GDTEST_PARSER_ERROR
-Only one underscore can be used as a numeric separator.
+Multiple underscores cannot be adjacent in a numeric literal.

modules/gdscript/tests/scripts/parser/errors/multiple_number_separators_after_decimal.gd

@@ -0,0 +1,3 @@
+func test():
+	# Number separators may not be placed right next to each other.
+	var _num = 123.45__67

modules/gdscript/tests/scripts/parser/errors/multiple_number_separators_after_decimal.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Multiple underscores cannot be adjacent in a numeric literal.