Missing `multi_memory` in types

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The multi_memory feature is not declared in the WasmFeatures interface in the index.d.ts file, nor is it declared anywhere in the documentation. This option does work and is specifiable.

Also, the https://github.com/WebAssembly/wabt/blob/main/src/feature.def link in index.d.ts should be replaced with https://github.com/WebAssembly/wabt/blob/main/include/wabt/feature.def.