function getContext(key) {
	const context_map = get_or_init_context_map('getContext');
	const result = /** @type {T} */ (context_map.get(key));
	return result;
}
Retrieves the context that belongs to the closest parent component with the specified `key`.
Must be called during component initialisation.

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