[context] createContext function recommendation

[augustjk]

The current recommendation for createContext has

community-protocols/proposals/context.md

Lines 114 to 121 in 952f158

	### `createContext` functions
	It is recommended that TypeScript implementations provide a `createContext()` function which is used to create a `Context`. This function can just cast to a `Context`:
	```ts
	export const createContext = <ValueType>(key: unknown) =>
	key as Context<typeof key, ValueType>;
	```

This effectively makes typeof key to always be unknown.

Lit's implementation has

export function createContext<ValueType, K = unknown>(key: K) {
  return key as Context<K, ValueType>;
}

which allows taking an type argument for the context key. In order to take advantage of this though, both type arguments must be provided. If only ValueType is provided, K defaults to unknown as TypeScript does not do partial inference of type arguments.

Now in practice, I don't think having an explicit KeyType be provided to Context is quite necessary. The extraction of the ValueType does not require it:

community-protocols/proposals/context.md

Lines 101 to 104 in 952f158

	```ts
	export type ContextType<Key extends Context<unknown, unknown>> =
	Key extends Context<unknown, infer ValueType> ? ValueType : never;
	```

I'm a bit torn as it feels correct to allow explicitly typing the KeyType as in the Lit implementation, but if it's functionally moot, that creates confusion like lit/lit#4601

One issue with having KeyType be unknown is that it can look confusing. With unknown & {__context: ValueType}, it just looks like {__context: ValueType} in type intellisense.
e.g.

const key = Symbol('foo');
const foo = createContext<{foo: string}>(key);
//    ^? const foo: {
//           __context__: {
//               foo: string;
//           };
//       }