Support for numeric units

[MewPurPur]

Feature Description

Numeric attributes often have units: https://www.w3.org/TR/SVG11/coords.html

In GodSVG, units are not only not handled, but they outright break without warning, since numeric fields do expect a specific format. I'm shocked not to have received any reports about this yet, so let's get ahead of that.

I have no concrete implementation details yet. I'll add things below as I learn:

Percentages are a wild card

The SVG's width and height can be defined in centimeters. In that case, percentages use real world units.

Also, attributes already have 4 different percentage handlings, but they seem to closely align with what units are allowed too, so the percentage handling and number ranges may be merged into a single setting. But probably not. I don't know--

Units are not accepted universally

Accepts \ Range	Any	Only positive	Fraction
Units
% and units	x1, x2, x, cx, fx, y1, y2, y, cy, fy stroke-dashoffset	rx, ry, r, width, height, stroke-width
%			opacity, fill-opacity, stroke-opacity, stop-opacity, offset
None		stroke-miterlimit

(some of the above might not be implemented yet, they are for future reference)