add same_line and anchored movements

[pantos9000]

This PR picks up the stale PR #4204. Big thanks to @mitsuhiko for his PR. Seeing the old changes made it really easy to jump right into the code 😎 👍

This PR adds two kinds of new movement sets: same_line and anchored movement.

For more info, check the old PR #4204.

same_line movement

Commands added:

• move_same_line_char_left
• move_same_line_char_right
• extend_same_line_char_left
• extend_same_line_char_right
• append_mode_same_line

These new commands move cursors, while making them stay in the same line. So if a cursor would wrap around into another line, instead it won't move and stay at its current position.

anchored movement

Commands added:

• move_anchored_line_up
• move_anchored_line_down
• move_anchored_visual_line_up
• move_anchored_visual_line_down
• extend_anchored_line_up
• extend_anchored_line_down
• extend_anchored_visual_line_up
• extend_anchored_visual_line_down

These new commands move cursors vertically. A cursor will move depending on its position:

• If it is on a newline character of a non-empty line, the cursor will stay on newlines (i.e. on a line's last character).
• If it is on a non-newline character of a non-empty line, the cursor will try to avoid newline characters. It will move normally, but if it would end up on a newline, instead it will be moved one position left of it (i.e. the line's second to last character).
• If it is on the newline character of an empty line (that contains nothing except the newline character), the cursor will continue to move like before: If it stayed on newline before, it will continue to do so. Otherwise it will try to avoid them (except on empty lines).

Differences to the old PR

• Based on recent master
• Commands support soft-wrap / visual movement
• Added some unit tests
• Slightly changed command names
• Additional command append_mode_same_line (for consistency, as append is more or less move right + insert mode)

Remarks

Command / function names

I was not sure about the general naming - e.g. anchored I think is used somewhere else in the code in a different context. I guess it would be bad if this somehow collided, so you might want to keep that in mind during review. If anyone spots any ambiguities - I'm always open for improvements :)

Helper functions context

I created some functions that are only used locally, e.g. is_in_visual_empty_line(). In case you think it makes sense to have some of those in a more common context (core?), I can gladly move them.

Unit tests

I struggled a bit for the soft-wrap functionality, the corresponding tests might be too implementation-specific and break in the future, if the soft-wrap functionality is adjusted. Couldn't find a better way, so I hope the tests are okay like this.

Also I skipped the unit tests for Movement::Extend, as the functionality is exactly the same as with Movement::Move.

Docs

I don't know if these commands should be documented somewhere, e.g. in the book. If it makes sense, I can write a few describing words.

Only a single setting

If these movements should have only a single setting (i.e. a bool that can be set) then this would probably affect a bit the architecture. Not sure how it would be best to implement it, but I would maybe go for additional values in either helix_core::Movement or helix_core::Direction.

But I think the way it was implemented previously (and still is) allows for quite versatile configuration and fits nice into the existing architecture, so I would prefer it the way it is.

[pantos9000]

Found a bug in the anchored implementation, made the PR a draft.

[mitsuhiko]

Thank you so much for picking this back up.