Add getters for WebIDL's dictionaries.

[alexlapa]

Motivation

Currently web-sys wrappers do not have any getters for WebIDL dictionaries (e.g. MediaStreamConstraints).

Such fields can be accessed using js_sys::Reflect, which is quite awkward.

Proposed Solution

Add getters for WebIDL dictionaries fields.

Alternatives

Some generic getter like: fn get(name:&str) -> JsValue