add create_context helper

[ryanatkn]

update: refined the API in #57

Adds a create_context helper for better ergonomics with getContext/setContext.

See github.com/sveltejs/svelte/issues/8941 (ugh this backlink-spammed the Svelte issue, sigh, it didn't linkify github.com/sveltejs/svelte omitting the https:// so I then added issues/8941 and it did linkify, creating a spammy backlink - wrapping with backticks seems to be the best option here, <> doesn't help)

I originally tried a class implementation but it has some type limitations that the function create_context does not have because it leverages overloaded function signatures.

Examples: (notice the subtle type differences)

const required_with_fallback = create_context({fallback: () => new SomeThing()});
// {
//     get: () => SomeThing;
//     set: (value?: SomeThing) => SomeThing;
//}
const optional_and_may_get_undefined = create_context<SomeThing>({optional: true});
// {
//     get: () => SomeThing | undefined;
//     set: (value: SomeThing) => SomeThing;
// }
const required_and_throws_on_undefined = create_context<SomeThing>();
// {
//     get: (error_message?: string) => SomeThing;
//     set: (value: SomeThing) => SomeThing;
// }

I originally included an optional label to error messages, but it's not relevant if a fallback is provided other than being added to the context key symbol, which may not do anything useful.

Fixed the error_message type here - 56462f9 - since it's only valid in the fallbackless/nonoptional case.

Followup:

• maybe remove error_message?

This PR also cleans up some stuff that was moved to Moss - some of these related things will probably be published as components later.