See neovim/neovim#39916 and my comment (neovim/neovim#39916 (comment)). This PR implements the 2nd solution.

Current foldcolumn behavior when fdc=3:

-  line 1
|- line 2
2- line 3
23 line 4

Proposed behavior:

-  line 1
|- line 2
2- line 3
2| line 4

And with fdc=4:

-   line 1
|-  line 2
||- line 3
2|- line 4
2|| line 5

On lines where we can't draw all the fold levels, this would turn the first column into a spill indicator. Thus 2| would mean there are two fold levels that are "collapsed".

IMO this is a better use of the limited space than always displaying [n-1][n], and is a bit clearer visually.