Tighten word-spacing / whitespace in quasi proportional mode

[jul-sh]

The current word spacing in quasi proportional mode is pretty wide. This can make it harder to read. Proportional fonts usually have narrower word spacing. Since monospace constraints don't apply in quasi proportional mode, I suggest reducing the whitespace width, either by default, or by allowing users to specify a variant.

I created a small demo to show the difference: https://jul-sh.github.io/demo-word-spacing-iosevka/.

I currently post process my own version of iosevka to reduce whitespace width. It'd be great to upstream the option.

[jmcwilliams403]

Maybe this can be approached by shrinking side bearings under extended width, or enabling texture healing under quasi-proportional, or a combination of both.

[jul-sh]

enabling texture healing under quasi-proportional

tried this, didn't make much of a difference. Shrinking the whitespace char was the only thing that got me there pretty much