feat: adjusted "fetch mathlib cache for ..." commands

[mhuisi]

This PR splits the 'Fetch Mathlib Build Cache For Current Imports' command into three:

1. 'Fetch Mathlib Build Cache For Open Files': Displayed in the forall menu and the command prompt. Displays a quick select dialog where all open files are selected by default and allows configuring the set of files to fetch the cache for.
2. 'Fetch Mathlib Build Cache For All Open Files': Only displayed in the command prompt (can also be bound to a keyboard shortcut). Fetches the cache for all open files without displaying a selection dialog.
3. 'Fetch Mathlib Build Cache For Current File': Only displayed in the command prompt (can also be bound to a keyboard shortcut). Fetches the cache for the currently displayed file without displaying a selection dialog. Re-uses the command ID of the previous 'Fetch Mathlib Build Cache For Current Imports' command, which had the same functionality.

Also ensures that these commands can be used downstream of Mathlib, which was added in mathlib4#21666.