Merge `xilem_html` and `xilem_svg` as `xilem_web` #28

Job	Run time
cargo clippy + test (windows-latest)	3m 40s
cargo doc (windows-latest)	3m 37s
cargo fmt	11s
cargo clippy + test (macos-latest)	3m 22s
cargo doc (macos-latest)	2m 29s
cargo clippy + test (ubuntu-latest)	2m 42s
cargo doc (ubuntu-latest)	2m 44s
	18m 45s

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