Option to not add units of measure

[stoyan]

Currently

a{width:1}

becomes

a{width:1px}

which breaks my UI by turning invalid CSS into a valid one.

(Of course the right thing to do is to fix my origin code to not contain invalid CSS. But in a large project that may take a while and hurt the adoption of LightningCSS.)

I think in this case the correct behavior would be to drop the property completely as its value is incorrect.