svg: support word-spacing

Parse word-spacing from presentation attributes and CSS, including
inheritance, normal, and font-relative units.

SVG word-spacing adds a fixed offset at word separators, while
Text::spacing() only scales glyph advances. Derive the scale from
measured advances and the number of spaces.

This preserves the total offset but distributes it across glyph
advances. Exact support requires a text layout API that accepts absolute
word-separator offsets after font positioning.

issues: #4107