## Description

The PR updates `SingleActivator` in order to add a parameter for specifying that a shortcut depends on <kbd>NumLock</kbd> key state. 

Somewhat similarly to what is possible with common modifiers expect that a boolean is not enough in this case because: by default, a shortcut should ignore the <kbd>NumLock</kbd> state and it should be possible to define shortcuts that require <kbd>NumLock</kbd> to be locked and other that require it to be unlocked.

@gspencergoog I considered defining a new `ShortcutActivator` implementation for this, but I thinks that adding the feature directly to `SingleActivator` results in a cleaner API.

## Related Issue

Fixes https://github.com/flutter/flutter/issues/145144
Preparation for https://github.com/flutter/flutter/issues/144936

## Tests

Adds 3 tests.