Support App Intents and app shortcuts

macOS 13 Ventura added a new API called App Intents (https://developer.apple.com/documentation/appintents). MacVim should support that to allow for integration into other apps and automation.

TODO: Think about what kind of intents to expose. We already exposes 2 macOS "Services" (a pretty old API) today: one for opening MacVim in a specific folder, and another for opening MacVim from a particular selected text. We can add those two to App Intents as well.

Other than those two, some ideas for feature set:

• create/open files
• query/filter opened files, properties, plugins, and more.
• process files with Vim script
• run generic Vim script

A lot of the above can already be done with Vim's +clientserver and using mvim -c in a ZSH script. Adding dedicated shortcuts does seem nicer as it would provide an easier and more structured interfaces for stringing together commands in Shortcuts.app. But we should make sure that whatever we add does provide genuine value to the user.

In addition to app intents (which could be used to compose shortcuts), we should provide default app shortcuts that can be used as is.

Also, interestingly, it doesn't seem like a lot of apps have added support for this, and exactly 0 text editors / IDEs seem to support it.

Start with Apple’s App Intents documentation and MacVim’s existing macOS Services for opening a folder or selected text. The scope is not settled: done requires an agreed set of intents plus default shortcuts that provide genuine value, with support for the selected workflows.