Merge `xilem_html` and `xilem_svg` as `xilem_web` #23

Job	Run time
cargo doc (windows-latest)	3m 49s
cargo clippy + test (windows-latest)	2m 14s
cargo doc (macos-latest)	1m 56s
cargo fmt	17s
cargo clippy + test (macos-latest)	1m 10s
cargo doc (ubuntu-latest)	2m 25s
cargo clippy + test (ubuntu-latest)	2m 5s
	13m 56s

Provide feedback