[librt] Add minimal librt.strings.StringWriter class (experimental) (#20588)

It can be used to build a string from code points (via `append` method)
and strings (via `write`). It's similar to `BytesWriter`, but it has to
keep track of the kind of string being created (1/2/4 bytes per code
point).

I've optimized `append` but there are no mypyc primitives yet, so
performance is still bad. I'll add mypyc primitives in a follow-up PR
and do further optimizations as needed.

There is no `truncate` or `__setitem__`, unlike `BytesWriter`, since
these could require the recomputation of kind, and this would be
inefficient. My current thinking is that we won't support these.

I used LLM assist, especially for tests, but manually reviewed all
changes.

Author: JukkaL

mypy/typeshed/stubs/librt/librt/strings.pyi

@@ -1,13 +1,21 @@
 from typing import final
 
-from mypy_extensions import i64, u8
+from mypy_extensions import i64, i32, u8
 
 @final
 class BytesWriter:
     def append(self, /, x: int) -> None: ...
-    def write(self, /, b: bytes | bytearray) -> None: ...
+    def write(self, b: bytes | bytearray, /) -> None: ...
     def getvalue(self) -> bytes: ...
-    def truncate(self, /, size: i64) -> None: ...
+    def truncate(self, size: i64, /) -> None: ...
     def __len__(self) -> i64: ...
-    def __getitem__(self, /, i: i64) -> u8: ...
-    def __setitem__(self, /, i: i64, x: u8) -> None: ...
+    def __getitem__(self, i: i64, /) -> u8: ...
+    def __setitem__(self, i: i64, x: u8, /) -> None: ...
+
+@final
+class StringWriter:
+    def append(self, x: int, /) -> None: ...
+    def write(self, s: str, /) -> None: ...
+    def getvalue(self) -> str: ...
+    def __len__(self) -> i64: ...
+    def __getitem__(self, i: i64, /) -> i32: ...