Adds support for right-hand gutters and Scrollbar gutter item

[jakesactualface]

Relates to #2210

First time contributor, so please let me know if I'm missing anything!

Overview

• Adds support for gutter items rendered on the right-hand side of the view
• Adds support for a new "scrollbar" gutter item
  • Note: this item is not added to the default configuration for the left- or right-hand gutter

Architectural Changes

• A gutters-right configuration option is exposed as a sibling to the existing gutters option
• View now accepts an additional gutters_right parameter as a GutterConfig object
• Gutter item functions are now called for each visible line in the view, rather than stopping at the last line of document contents
  • See last_line initialization within render_gutter_item for more context
• A Scrollbar variant was added to the GutterType enum

Design feedback needed

This gutters vs. gutters-right implementation seemed like the easiest approach (and was also passive to the existing config file setup), but I quite like how the editor.statusline configuration is subdivided into left, center, and right partitions.

• Should we wrap gutter configuration into similar left and right partitions?

[pascalkuthe]

This is not really a fault of this PR but there is currently a larger change to editor rendering underway in #5420.
This PR heavily conflicts with #5420 and needs to be rebased on that once that is merged. #5420 fundamentally changes how gutters are rendered (or really all editor ui) and especially the following change will be problematic:

Gutter item functions are now called for each visible line in the view, rather than stopping at the last line of document contents

[jakesactualface]

@pascalkuthe no worries! I'm fine with rebasing onto your changes. I gave it a try preemptively based on the current contents of your PR and it seems like the right gutter is still rendering correctly, though I'll have to adjust the math for the scrollbar position.

In the meantime, should I convert this PR to a draft until #5420 is merged? Or is there a different way you'd like me to proceed?

[pascalkuthe]

It would have been fine to leave the PR as is, we have a label that we put in PRs that are based on other.

Howbwr #5420 has been merged into master today so you can just rebase off master

[jakesactualface]

It would have been fine to leave the PR as is

Sorry, I meant that I performed a quick rebase locally, just to see how big of a conflict I was in store for.

I still have some cleanup and validation to do before I'm ready to push that rebase here, but I'll try to get that done within the next few days. Thank you for the heads-up!

[jakesactualface]

I have rebased my changes onto a fresh copy of master, as well as divided them up between 3 commits. This was done to give some logical separation between the main ideas being attempted:

1. Add support for a gutter on the right-hand side of the view
2. Add support for a "scrollbar" gutter item
3. Enable gutter rendering for lines within the viewport but beyond EOF

The changes in Point 3 above (localized for now in the 3rd commit on this PR) could definitely benefit from some more-experienced eyes than my own, if that functionality is going to be considered for merge. I'm still fairly new to Rust and I suspect there's a better way for me to be tracking the viewport's position during the text rendering iteration.

I've included some screenshots below for an overview of how this branch looks in use, in case that makes things easier for anyone interested.

Line numbers enabled for right-hand gutter (left-hand gutter is explicitly set to default layout):

Right-hand gutter set to "scrollbar", "spacer", "line-numbers":

Same file and setup, with the viewport scrolled beyond EOF:

[sudormrfbin]

We already use the half block character for the completion popup scrollbar, so we could use the same here for consistency:

helix/helix-term/src/ui/popup.rs

Lines 277 to 281 in 15e07d4

	cell = &mut surface[(inner.right() - 1, inner.top() + i as u16)];
	cell.set_symbol("▐"); // right half block
	if scroll_line <= i && i < scroll_line + scroll_height {

[jakesactualface]

I originally shied away from that since there's nothing stopping someone from putting the scrollbar on the left, but it looks like the diff gutter is making a similar assumption and it looks better for it.

I'll add the change as suggested, but I'm open to discussing it if others have a different opinion.