Add count support in copy mode

[pizdoponi]

Is your feature request related to a problem? Please describe.
When in copy mode, I have to press a lot of arrow keys to get to a specific line (after getting as close as possible using Ctrl+u, M, etc.). And then if I want to select 8 lines, I have to hit the down arrow 8 times again, making it very cumbersome.

Describe the solution you'd like
Add count support to movements, especially for up and down movement. Similar to how 5j would move the cursor 5 lines down.

Describe alternatives you've considered
f, F, t, T work well for granular horizontal movement, but for vertical (aside from Ctrl-u, Ctrl+d, H, M, L for "big" movement) there is no way to specify by exactly how much one wants to move.

Additional context
A similar feature request had been requested in #4136 and the idea was elaborated on in #2988.