Support wasm-pack POST method in rust-playground

[titaneric]

rust-lang/rust-playground#374 mention support wasm rendering in rust-playground.
rust-lang/rust-playground#706 is the corresponding implementation

To support such feature in mdBook, we may

• Call their POST method and fetch result (JS and Wasm)
• Render the JS and Wasm in IFrame
• Differentiate the normal rust and wasm code, perhaps support new wasm className