Add `html_as_default_extension` option to `ServeDir` #1126

Job	Run time
check	46s
check-docs	42s
cargo-hack	1m 12s
cargo-public-api-crates	1m 7s
cargo-deny check (advisories)	29s
cargo-deny check (bans licenses sources)	19s
test-versions (stable)	1m 15s
test-versions (beta)	1m 12s
test-versions (nightly)	1m 7s
test-msrv	1m 14s
style	5s
	9m 28s

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