Context object for type editor hosts to attach metadata.
Some editors need things like information on where to put a react portal
(for tooltips and such). Normally, the default behavior is adequeate,
but for specialized cases (like rendering inside a popup from the parent
window) this is insufficient. This sort of information is inappropriate
to pass via properties to the generic host, so instead hosts can use this
context to describe that sort of information to type editor instances.