Merge `xilem_html` and `xilem_svg` as `xilem_web` #109

Job	Run time
cargo fmt	12s
cargo doc (windows-latest)	4m 49s
cargo clippy + test (windows-latest)	3m 2s
cargo doc (macos-latest)	2m 20s
cargo clippy + test (macos-latest)	4m 57s
cargo doc (ubuntu-latest)	1m 44s
cargo clippy + test (ubuntu-latest)	1m 50s
	18m 54s

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