function setContext(key, context) {
	const context_map = get_or_init_context_map('setContext');
	context_map.set(key, context);
	return context;
}
Associates an arbitrary `context` object with the current component and the specified `key`
and returns that object. The context is then available to children of the component
(including slotted content) with `getContext`.

Like lifecycle functions, this must be called during component initialisation.

@template T
@param {any} key
@param {T} context
@returns {T}