This commit introduces a WEB_SET_PROTOTYPE_FOR_INTERFACE macro that
caches the interface name in a local static FlyString. This means that
we only pay for FlyString-from-literal lookup once per browser lifetime
instead of every time the interface is instantiated.
This patch adds basic support for the SVG `<textPath>`, so it supports
placing text along a path, but none of the extra attributes for
controlling the layout of the text. This is enough to correctly display
the MDN example.