Add usersettings stub files (#10598)

stubs/usersettings/@tests/stubtest_allowlist.txt

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+usersettings.Settings.app_id
+usersettings.Settings.settings_directory
+usersettings.Settings.settings_file

stubs/usersettings/METADATA.toml

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+version = "1.1.*"
+upstream_repository = "https://github.com/glvnst/usersettings"

stubs/usersettings/usersettings.pyi

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+from typing import Any, TypeVar
+
+_S = TypeVar("_S")
+
+class Settings(dict[str, Any]):
+    app_id: str
+    settings_directory: str
+    settings_file: str
+    def __init__(self, app_id: str) -> None: ...
+    def add_setting(self, setting_name: str, setting_type: type[_S] = ..., default: _S | None = None) -> None: ...
+    def load_settings(self) -> None: ...
+    def save_settings(self) -> None: ...
+    def __getattr__(self, setting_name: str) -> Any: ...
+    def __setattr__(self, setting_name: str, setting_value: Any) -> None: ...