Better narrow fold column display options

[alercah]

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Is your feature request related something that is currently hard to do? Please describe.
Currently, foldcolumn forces a tradeoff between screen real estate and usability/aesthetics, as a narrower column uses numbers to indicate too many nested folds, which is ugly and not super useable. There is not really good options for users who want a very narrow column.

Describe the solution you'd like
I would like to request two enhancements:

1. Allow the fold and sign columns to be merged, displaying the signs if present, and otherwise the fold information.
2. Allow nesting multiple folds into a single column for the fold column, like this:

+
|
|
+
|
|
|
-
|
-

Here, I use - to mark the bottom of the fold, but a proper 90 angle should probably be used for more clarity.

Describe alternatives you've considered
I considered trying to use fillchars, but it doesn't seem to have an option to turn the numbers off.

Additional context
There is currently work under way to allow the column to automatically scale with code depth, but this doesn't actually solve this particular problem.

[kevinhwang91]

Same here, also expect this feature. However, I have hacked src/screen.c by myself.