add Shortcuts.app support

It would be a cool feature to add support for the iOS Shortcuts.app

For starters a simple "Send Message to $Room" would be great to automate some messages on the go.

[relevant docs](https://developer.apple.com/documentation/sirikit/adding_user_interactivity_with_siri_shortcuts_and_the_shortcuts_app)

Start with the linked Apple Shortcuts and SiriKit documentation to understand the supported integration points for iOS. Done means users can invoke a Shortcut that sends a message to a selected Matrix room; the issue does not name implementation files or tests.

Written by the indexing model from the issue text.