svg: support the font-weight attribute

Parse font-weight from presentation attributes and inline styles on text
and tspan, plus CSS rules on text and inherited container styles.

Support normal, bold, inherit, bolder, lighter, and numeric values from
100 through 900 in 100-step increments.

Map weights 100-500 to regular rendering and 600-900 to a same-color
synthetic outline for solid fills.

issue: #4107