[w3c/webcomponents] Forbid reentrance to `customElements.define` (#545)

The current spec defines the notion of a [*set of being-defined names*](https://html.spec.whatwg.org/#set-of-being-defined-names) and a [*set of being-defined constructors*](https://html.spec.whatwg.org/#set-of-being-defined-constructors) to deal with the situation in which `CustomElementsRegistry.prorotype.define` is called inside `Get` invocations; i.e. a re-entrant call to `define`.

However, this situation is more easily avoided if we just added a global flag indicating whether we're inside `define` or not.  I can't think of a situation in which one would need a re-entrant behavior in `define` since making a call to `define` invokes trapping `Get(constructor, "prototype")` or other `Get`s to callback and `observedAttributes`.

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