Systematic expansion of SVG builders

[johnpmayer]

Looks like the current list of builders is hand-curated. It's great that I can create a tag manually via ElementBuilder, but it's more verbose.

I'm imagine automatically generating the all of builders in the svg namespace by pulling from sort of metadata depot, similar to how web_sys uses WebIDL. Ideally, typed-html could use the same - I found their dodrio!() macro similarly limiting as it whitelists the elements.