Consolidate wasm features in config.rs

[alexcrichton]

Use the WASM3 fixed feature set which Wasmtime now implements by default.

[github-actions]

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

• If you added a new Config method, you wrote extensive documentation for
  it.

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  Short, simple summary sentence.
  
  More details. These details can be multiple paragraphs. There should be
  information about not just the method, but its parameters and results as
  well.
  
  Is this method fallible? If so, when can it return an error?
  
  Can this method panic? If so, when does it panic?
  
  # Example
  
  Optional example here.
  

• If you added a new Config method, or modified an existing one, you
  ensured that this configuration is exercised by the fuzz targets.

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• If you are enabling a configuration option by default, make sure that it
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[dicej]

Is this comment still accurate and relevant given the change from WasmFeatures::WASM2 to WasmFeatures::empty()?