Opt-in support for sign-extend (#339)

Allows users to do the following to enable sign-extension-ops:

```
r := wazero.NewRuntimeWithConfig(wazero.NewRuntimeConfig().WithFeatureSignExtensionOps(true))
```

Resolves #66

Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>
Co-authored-by: Adrian Cole <adrian@tetrate.io>

config_test.go

@@ -23,6 +23,14 @@ func TestRuntimeConfig_Features(t *testing.T) {
 				return c.WithFeatureMutableGlobal(v)
 			},
 		},
+		{
+			name:          "sign-extension-ops",
+			feature:       internalwasm.FeatureSignExtensionOps,
+			expectDefault: false,
+			setFeature: func(c *RuntimeConfig, v bool) *RuntimeConfig {
+				return c.WithFeatureSignExtensionOps(v)
+			},
+		},
 	}
 
 	for _, tt := range tests {