Implement getters for WebIDL dictionaries #1985

Job	Run time
Run wasm-bindgen crate tests (unix)	4m 26s
Check	1m 6s
Clippy	2m 17s
Verify that web-sys is compiled correctly	47s
Rustfmt	6s
Run js-sys crate tests	2m 24s
Run native tests	1m 46s
Run wasm-bindgen crate tests with multithreading enabled	1m 27s
Test TypeScript output of wasm-bindgen	1m 24s
Run web-sys crate tests	9m 37s
build_benchmarks	56s
Run wasm-bindgen-webidl crate tests	1m 29s
build_examples	3m 53s
build_nightly	1m 34s
dist_linux_aarch64_gnu	2m 41s
dist_linux_x86_64_musl	2m 23s
dist_macos_aarch64	1m 6s
dist_macos_x86_64	1m 22s
dist_windows	3m 43s
doc_api	1m 12s
Build and test the deno example	48s
doc_book	4s
Run UI compile-fail tests	43s
test_examples	1m 53s
deploy	32s
	49m 39s

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