[VIM-3731] Add support for "jump to previous/next lowercase mark".

[sdoerner]

Fixes VIM-3731.

Let me know if I should do anything else with Mark.PositionSorter. The two data structures are in different packages, so I wasn't sure if sharing the code would be desired or where you'd want the shared code to live.

[mbouron]

For what it's worth, I've been running this PR applied on top of latest master for the last few days and it works as expected.

[lippfi]

For what it's worth, I've been running this PR applied on top of latest master for the last few days and it works as expected.

I've just checked out the PR to recheck my comment to be sure
Nope, it doesn't
Set multiple marks at the same place and then press ]`

Expected result: caret is moved to the next mark
Actual result: caret is stuck no matter how many times you press ]`

[sdoerner]

I've just checked out the PR to recheck my comment to be sure Nope, it doesn't Set multiple marks at the same place and then press ]`

Expected result: caret is moved to the next mark
Actual result: caret is stuck no matter how many times you press ]`

Pushed an update to the PR with a fix.

[lippfi]

Thank you for your work!
And thank you one more time for the tests, the number of tests is impressive.