Add narrow no-break space to `editor.whitespace`

[Iorvethe]

On the bépo keyboard layout, the narrow no-break space [1] is very easy to inadvertently type and having it as highlighted would be very helpful to avoid hard-to-track compile errors.

[1] https://symbl.cc/en/202F/

[archseer]

Is this is the same character as   in HTML?

[Iorvethe]

I believe it’s a different character, namely the &nnbsp; one.

[quentin-bettoum]

I would also like to have it highlighted. I will attempt a PR.