Add support for async derived context functions

[captbaritone]

Summary

This PR adds support for derived context functions that return Promise<T>, enabling async operations in derived context resolution.

Example

/** @gqlContext */
type Ctx = { db: DB };

type UserContext = { user: User };

/** @gqlContext */
export async function getUserContext(ctx: Ctx): Promise<UserContext> {
  const user = await ctx.db.getCurrentUser();
  return { user };
}

/** @gqlQueryField */
export function me(userCtx: UserContext): User {
  return userCtx.user;
}

Generates:

async resolve(_source, _args, context) {
  return queryMeResolver(await getUserContext(context));
}

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[captbaritone]

Fixes #204