Added visual indentation line guides for text editors

[EricEzaM]

I have found it a bit difficult to know exactly in which indentation 'context' I am in when working on code (especially long code blocks, sometimes spanning more than 1 "page"). This attempts to resolve that issue. I know you can draw tabs (and spaces) but it does not 100% solve the issue, and I find that drawing tabs makes too much visual clutter.

This give a solution found in most common IDE applications. There are editor settings for

• On/off indent guides
• On/off Highlight active indent guide
• Colors

Where/when to draw lines is based on the 'folding' mechanism already in the editor.

My first time adding editor settings so please check I did it right :) Thanks

Showcase

Before

[Calinou]

For future reference, I attempted this in #20725 but it didn't work when scrolling.

In my PR, I used a more subtle color for indent guides (Color(0.5, 0.5, 0.5, 0.31)), which you could borrow in your PR as well. I think the current color is a bit too prominent, as indent guides are generally considered to be "secondary" information. That said, I like the idea of making the "current" indent level more contrasted (I'd still tone it down a bit).

[EricEzaM]

@Calinou that's true, looking at this again a few days later, the colors are a bit too prominent.

[EricEzaM]

This is probably better to be implemented at some point in the future, maybe 4.1, when the api for script/code/text editor is finalised.

Closing for now, and would submit as a separate PR in the future if I were still to implement.