The external encoding must be set for CRuby WASI

* Otherwise it's US-ASCII and doesn't display correctly any string with
  non-ASCII characters, e.g. p "été" becomes "\u00E9t\u00E9".

Author: eregon

app/ruby_engine/cruby_wasi.rb

@@ -102,6 +102,8 @@ def run(source)
         `vm.setInstance(wasmInstance)`.await
         `wasi.setMemory(wasmInstance.exports.memory)`
         `vm.initialize()`
+        set_external_encoding = "Encoding.default_external = Encoding::UTF_8"
+        `vm.eval(set_external_encoding)`
       end
 
       yield `vm.eval(source).toString()`