Vim emulation: add "o", "O" commands

[mcclurgm]

Implements a basic form of the o and O commands in vim. It adds a new line before or after the current, navigates to that line, and enters insert mode.

As with #712 (and the other insertion commands), this doesn't properly work with numbers. It won't repeat the insert multiple times, only once.

[jeremypw]

This works as expected but some simplification is possible.

[mcclurgm]

Ok, I fixed your requests. There's one big issue remaining here that I just discovered, that the MovementStep.PARAGRAPH_START goes to the beginning of the line excluding whitespace. So the shift-O command needs some more work. I'll see if I can fix that.

[mcclurgm]

Since the effect is pretty much the same as inserting a newline after the 0 motion, I'm going to wait until we figure that out and then follow that solution here.

[mcclurgm]

One more thought I just had. This doesn't respect the current line ending setting. I guess I'll have to fix that too… (Do we have a line ending setting? It's not exposed to the user, but that doesn't mean a ton necessarily)

[jeremypw]

Need someone to fix the flatpak CI before this can be merged.

[jeremypw]

I think any issues with non-standard line-endings can be dealt with, if required, later. Code is focused on code development on Linux.