Add narrow no-break space support

[quentin-bettoum]

The goal of this PR is to support the rendering of narrow no-break spaces as asked in #6640.

Note that I'm not proficient in Rust, but since it looked simple, I gave it a try by looking at what have been done for regular nbsp.

The tests passes and the feature works using cargo run.

I don't know what's better between having a dedicated editor.whitespace.render option for nnbsp or including it in the existing nbsp parameter.

Currently, the new config parameters would look like this:

[editor.whitespace.render]
space = "all"
nbsp = "all"
nnbsp = "all"
tab = "all"
newline = "none"

[editor.whitespace.characters]
space = "·"
nbsp = "⍽"
nnbsp = "␣"
tab = "→"
newline = "⏎"
tabpad = "·"