interfaces: add a Protocol class for `scmutil.status`

I initially tried moving this to the `interfaces` package, both to have more
cleanly defined interfaces (interfaces shouldn't have to reach into
implementation files for their type info), and because importing `mercurial.ui`
either directly or indirectly into `interfaces.repository` causes a situation
where pytype stops inferring the type for `revlogutils.constants` that are
imported by `revlog`.  (Likely this is caused by a cycle.  The `dirstate`
interface already imports `scmutil`, which in turn imports `ui`, so the
`repository` interface module importing the `dirstate` interface module as part
of converting those classes to Protocol classes will trigger the issue.)

I gave up on moving the class because `scmutil.status` depends on `stringutil`,
which has a surprisingly long tail of dependencies.  In any event, a standalone
Protocol class might help with the Rust code.

mercurial/interfaces/status.py

@@ -0,0 +1,49
+# status.py - Type annotations for status related objects
+#
+# Copyright Matt Harbison <[e-mail]>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import annotations
+
+from typing import (
+    Iterator,
+    Protocol,
+)
+
+
+class Status(Protocol):
+    """Struct with a list of files per status.
+
+    The 'deleted', 'unknown' and 'ignored' properties are only
+    relevant to the working copy.
+    """
+
+    modified: list[bytes]
+    """The list of files with modifications."""
+
+    added: list[bytes]
+    """The list of files that started being tracked."""
+
+    removed: list[bytes]
+    """The list of files that stopped being tracked."""
+
+    deleted: list[bytes]
+    """The list of files in the working directory that are deleted from the
+    file system (but not in the removed state)."""
+
+    unknown: list[bytes]
+    """The list of files in the working directory that are not tracked."""
+
+    ignored: list[bytes]
+    """The list of files in the working directory that are ignored."""
+
+    clean: list[bytes]
+    """The list of files that are not in any other state."""
+
+    def __iter__(self) -> Iterator[list[bytes]]:
+        """Iterates over each of the categories of file lists."""
+
+    def __repr__(self) -> str:
+        """Creates a string representation of the file lists."""