Generated types do not remove $ prefix from JSG_STRUCT members.

If a JSG_STRUCT has a member whose name is, say, `$class` in C++, the JavaScript name is `class`. The `$` is supposed to be stripped off.

The actual runtime does this correctly, but the generated types are not stripping the `$`.

An example is `DurableObjectFacetsStartupOptions` (an experimental type).