Add a method to manage copied files on iOS

[lindboe]

When the share extension needs to open the main app on iOS, the library currently makes a copy of any shared file, as the temporary copy that the share extension can access will not be available past the share extension's lifetime. However, those files are never cleaned up by the library, and users may not realize these copies are being made.

To help manage this problem, we'll create a new native method, deleteTempCopy, which will accept the file name as an argument, that apps can call when they're finished using the copied file on iOS. The method should exist on Android, for platform parity, but immediately return (does not need to be native code, a JS export for Android is fine -- but this may not work down the line with New Arch and codegen).

Acceptance criteria

• The method should be usable from JS in the main app
• If the file is not present or deleteable, the method should return a rejected promise
• Add a changelog entry
• Add documentation of what this method is for and how to use it

Out of scope:

• Adding code to help library users clear out existing temp copies. This is best done on a per-app basis with react-native-fs or similar.