test

Add `.codecov.yml` and README badges #12

Job	Run time
unit-tests / lint	31s
unit-tests / ios-unit	0s
unit-tests / api-breakage	4m 19s
unit-tests / linux-unit (swift:5.9-jammy)	26s
unit-tests / linux-unit (swift:5.10-noble)	31s
unit-tests / linux-unit (swift:6.0-noble)	2m 58s
unit-tests / macos-unit (macos-13, ~15.2)	15s
unit-tests / linux-unit (swiftlang/swift:nightly-main-jammy)	4m 4s
unit-tests / windows-unit	0s
unit-tests / musl	0s
unit-tests / macos-unit (macos-14, latest-stable)	2m 16s
	15m 20s

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