Add @computed.readOnly and @computed.volatile

[pzuraq]

Currently, computed properties made with decorators are readOnly by default, and there is no way to have them be marked as volatile because undecorated native getters/setters are preferred. In order to create a general purpose codemod, we need to make sure that we have the exact same semantics as current computeds do. To do this, we need to make computeds clobberable by default, and add methods for invoking readOnly/volatile behavior.

[pzuraq]

Added in v2.5.0