Add WebIDL definitions relating to `Popover API`. #2170

Job	Run time
Check	1m 4s
Clippy	6m 34s
Rustfmt	13s
Verify that web-sys is compiled correctly	52s
Run js-sys crate tests	2m 52s
Build and test the deno example	55s
Run native tests	1m 46s
Test TypeScript output of wasm-bindgen	1m 18s
Run wasm-bindgen crate tests with multithreading enabled	1m 25s
Run wasm-bindgen crate tests (unix)	6m 20s
Run web-sys crate tests	7m 27s
Run wasm-bindgen-webidl crate tests	2m 7s
build_benchmarks	1m 4s
build_examples	3m 51s
build_nightly	1m 31s
dist_linux_aarch64_gnu	2m 47s
dist_linux_x86_64_musl	2m 24s
dist_macos_aarch64	1m 9s
dist_macos_x86_64	1m 10s
dist_windows	2m 29s
doc_api	1m 13s
doc_book	5s
Run UI compile-fail tests	43s
test_examples	2m 1s
deploy	34s
	53m 54s

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