Extmarks api: allow for gravity

[chentoast]

Allows for extmark gravity to be set.

An example: suppose we have the following text, with an extmark placed where the pipe is:

asdf|asdf

normally, if we type (gravity = "right"), the extmark will move with the cursor:

asdfhello|asdf

but with gravity = "left", the extmark will stay put:

asdf|helloasdf.

To decide: do we want to allow gravity for the end_col and end_row of extmarks as well? Would be useful to know of potential use cases where this would be needed.

[chentoast]

better to make it a bool, there will only be two values.

There was a helpful discussion about this in #12249. In the end, I felt that two options seemed the best to me:

{gravity = "right"}, {gravity = "left"}

or

{right_gravity = true}, {right_gravity = false}

Either option is fine with me. Once we decide on which one to go with, I'll add the corresponding option for end_gravity.

[bfredl]

right_gravity=true is better.

[runiq]

What about RTL (if it's ever going to be supported with extmarks, that is)? Would it make sense for RTL text to have right_gravity=false by default?

[runiq]

Friendly reminder: #12742 is still open, if you find anything useful in there, feel free to steal :)

[bfredl]

What about RTL (if it's ever going to be supported with extmarks, that is)? Would it make sense for RTL text to have right_gravity=false by default?

RTL is a display feature (that UI:s are gonna handle in differerent ways), it better not affect editing semantics in the core layer.

[chentoast]

@bfredl whenever you have the time, do you mind reviewing this? I think its pretty much ready for merge.

[bfredl]

I agree, merged :)