Implement getters for WebIDL dictionaries #1983

Job	Run time
build_examples	3m 53s
build_benchmarks	59s
build_nightly	1m 39s
Check	58s
Verify that web-sys is compiled correctly	44s
Clippy	2m 24s
dist_linux_aarch64_gnu	2m 47s
dist_linux_x86_64_musl	2m 22s
dist_macos_aarch64	1m 5s
dist_windows	3m 3s
dist_macos_x86_64	1m 6s
doc_api	1m 11s
Rustfmt	11s
Build and test the deno example	1m 1s
doc_book	5s
Run js-sys crate tests	2m 13s
Run native tests	1m 45s
Run wasm-bindgen crate tests with multithreading enabled	1m 24s
Test TypeScript output of wasm-bindgen	1m 22s
Run UI compile-fail tests	41s
Run wasm-bindgen crate tests (unix)	4m 31s
Run web-sys crate tests	5m 48s
Run wasm-bindgen-webidl crate tests	50s
test_examples	1m 49s
deploy	36s
	44m 27s

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