Implement getters for WebIDL dictionaries #1986

Job	Run time	Billable time
Run wasm-bindgen crate tests with multithreading enabled	1m 34s	0s
Rustfmt	7s	0s
Check	1m 0s	0s
Clippy	2m 17s	0s
Run wasm-bindgen crate tests (unix)	6m 29s	0s
build_examples	4m 1s	0s
build_nightly	1m 38s	0s
build_benchmarks	47s	0s
Verify that web-sys is compiled correctly	47s	0s
dist_linux_x86_64_musl	2m 26s	0s
dist_macos_aarch64	1m 6s	0s
dist_linux_aarch64_gnu	2m 44s	0s
dist_macos_x86_64	1m 19s	0s
dist_windows	4m 5s	0s
doc_api	1m 10s	0s
doc_book	4s	0s
Build and test the deno example	45s	0s
Run js-sys crate tests	2m 12s	0s
Run native tests	1m 43s	0s
Test TypeScript output of wasm-bindgen	1m 17s	0s
Run UI compile-fail tests	40s	0s
Run web-sys crate tests	6m 29s	0s
Run wasm-bindgen-webidl crate tests	1m 25s	0s
test_examples	1m 47s	0s
deploy	38s	0s
	48m 30s	0s

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