Implement getters for WebIDL dictionaries #1988

Job	Run time	Billable time
Check	59s	0s
build_examples	3m 59s	0s
build_benchmarks	53s	0s
build_nightly	1m 44s	0s
Verify that web-sys is compiled correctly	46s	0s
Clippy	6m 51s	0s
dist_linux_aarch64_gnu	2m 44s	0s
dist_linux_x86_64_musl	2m 16s	0s
dist_macos_aarch64	1m 7s	0s
dist_macos_x86_64	1m 8s	0s
dist_windows	3m 37s	0s
doc_api	1m 27s	0s
doc_book	9s	0s
Rustfmt	8s	0s
Build and test the deno example	46s	0s
Run js-sys crate tests	2m 15s	0s
Run native tests	1m 55s	0s
Run wasm-bindgen crate tests with multithreading enabled	1m 35s	0s
Test TypeScript output of wasm-bindgen	1m 26s	0s
Run UI compile-fail tests	41s	0s
Run wasm-bindgen crate tests (unix)	8m 38s	0s
Run web-sys crate tests	7m 25s	0s
Run wasm-bindgen-webidl crate tests	2m 5s	0s
test_examples	1m 55s	0s
deploy	29s	0s
	56m 58s	0s

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