Fix cursor position bugs related to o and O

[sudormrfbin]

• O at the beginning of file didn't move cursor
• o and O messed up cursor position with multiple cursors

Fixes #127

[pickfire]

Nice, I was trying this out the other day but I didn't know you need to do change_by_selection. Seemed bad that we need to keep a state around. The bug is fixed but the behavior seemed to be different from kakoune and vim. 10Oa now does not write 10 a but instead just one a 10 lines above, I think this is a bit surprising.

[sudormrfbin]

Nice, I was trying this out the other day but I didn't know you need to do change_by_selection

The problem doesn't stem from using change instead of change_by_selection though, the issue is with properly calculating how much an edit needs to be offset by prior edits in the same transaction. As @archseer said on Matrix it'd be cleaner of we could handle this in change_by_selection itself.

10Oa now does not write 10 a but instead just one a 10 lines above, I think this is a bit surprising.

It's deviant from vim and kakoune yeah but do we really need to keep that behavior in helix ? It feels at odds with the multiple cursor philosophy (this commit didn't change this behavior though, it previously did not write 10 a's either).

[archseer]

10Oa now does not write 10 a but instead just one a 10 lines above, I think this is a bit surprising.

This was already a problem before this PR, the implementation simply doesn't do that yet.

do we really need to keep that behavior in helix ?

I do want this, 10o should make 10 lines and place a cursor on each. It's an easy way to start writing multiple lines with multiple cursors.

[sudormrfbin]

How should counts before i, I, etc be handled ? In vim 10i<text>Esc inserts <text> on the same line 10 times, which isn't very useful. IMO something more useful would be putting cursors on the current line and the next 9 lines in the same column. Same for 10I but in the beginning of each line.

[archseer]

I think it's OK if counts on i are just ignored.

Do you want to do the change for 10o here, or should that be a follow-up?

[sudormrfbin]

I'll add it as a separate commit in this PR.

Edit: Added counts

[archseer]

Verified on local. Thanks!