Draft of fix for ChainableAttributeSpec being assigned to SharedType type attribute

[remaininlight]

When a shared type is used on a record in version 3.0.8 an error is thrown when assignment to it is attempted:

Uncaught TypeError: Right-hand side of 'instanceof' is not callable
at SharedType.convert (shared.ts:90)
at SharedType.doUpdate (shared.ts:56)
at setAttribute (updates.ts:56)
at AppState.set (any.ts:108)
at onEdit (main.jsx:69)
at HTMLUnknownElement.callCallback (react-dom.development.js:540)
at Object.invokeGuardedCallbackDev (react-dom.development.js:579)
at Object.invokeGuardedCallback (react-dom.development.js:436)
at Object.invokeGuardedCallbackAndCatchFirstError (react-dom.development.js:450)
at executeDispatch (react-dom.development.js:834)

This seems to be because in these cases the type attribute on SharedType is a ChainableAttributeSpec rather than a Transactional type. This pull request is very crude, largely bypassing the type system, I'm not precisely sure how the type inheritance hierarchy is supposed to fit together - please advise how this PR could be improved.

You should be able to replicate the bug using an example such as this and assigning to the editing attribute:

@define class AppState extends Record {
static attributes = {
editing : shared(type(User)),
}
}