[10.x] Support human-friendly text for file size

[jxxe]

Allows declaring file size limits as 10mb instead of 10000 when validating file sizes.

$data = $request->validate([
    // ...
    'image' => 'required|image|max:10mb'
]);

I can never remember which unit file size limits are in, and since I'm often working in megabytes, the number of zeroes makes it hard to comprehend at a glance.

This converts numbers suffixed with kb, mb, gb, or tb (why not) into a number of kilobytes. Numbers without units work just as before.

[jxxe]

Actually, since I changed some arguments names (though not their order), this is a breaking change if people use named arguments. Should I change this to merge to 11.x?