Add Promisable type

[kainiedziela]

Fixes #40
TypeScript proposal: #31394

A type for values that can either be a promise or it's return value.

type Promisable<T> = T | PromiseLike<T>;

Example

async function logger(getLogEntry: () => Awaitable<string>): Promise<void> {
    const entry = await getLogEntry();
    console.log(entry);
}

logger(() => 'foo');
logger(() => Promise.resolve('bar'));

Use cases

• A function accepts a callback that may either return a value synchronously or may return a promised value.
• Specifically this type could be the return type of Promise.then(), Promise.catch() and Promise.finally() callbacks.