Merge `xilem_html` and `xilem_svg` as `xilem_web` #24

Job	Run time
cargo fmt	15s
cargo clippy + test (windows-latest)	2m 8s
cargo doc (windows-latest)	3m 18s
cargo clippy + test (macos-latest)	1m 33s
cargo doc (macos-latest)	48s
cargo clippy + test (ubuntu-latest)	1m 10s
cargo doc (ubuntu-latest)	36s
	9m 48s

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