string: Add missing FeatureNew tags to methods

There are some new(er) methods that have not version reference, so add
the missing ones in order to be properly notified when targetting older
meson versions.

Co-authored-by: Tristan Partin <tristan@partin.io>

mesonbuild/interpreter/primitives/string.py

@@ -117,6 +117,7 @@ class StringHolder(ObjectHolder[str]):
         return self.held_object.join(args[0])
 
     @noKwargs
+    @FeatureNew('str.replace', '0.58.0')
     @typed_pos_args('str.replace', str, str)
     def replace_method(self, args: T.Tuple[str, str], kwargs: TYPE_kwargs) -> str:
         return self.held_object.replace(args[0], args[1])
@@ -129,9 +130,12 @@ class StringHolder(ObjectHolder[str]):
     @noKwargs
     @typed_pos_args('str.strip', optargs=[str])
     def strip_method(self, args: T.Tuple[T.Optional[str]], kwargs: TYPE_kwargs) -> str:
+        if args[0]:
+            FeatureNew.single_use('str.strip with a positional argument', '0.43.0', self.subproject, location=self.current_node)
         return self.held_object.strip(args[0])
 
     @noKwargs
+    @FeatureNew('str.substring', '0.56.0')
     @typed_pos_args('str.substring', optargs=[int, int])
     def substring_method(self, args: T.Tuple[T.Optional[int], T.Optional[int]], kwargs: TYPE_kwargs) -> str:
         start = args[0] if args[0] is not None else 0