feat: support variable last_indent_marker width

[lukas-reineke]

With this, last_indent_marker can be more than one display cell wide. The difference is subtracted from the indent level.

indent = {
    indent_size = 4,
    last_indent_marker = "└──",
},

(I also fixed some typos, I can make a different PR for this if you want)

[cseickel]

Thanks!