fix: allow async option to dictate type returned

[UziTech]

Marked version: v13

Description

BREAKING CHANGE: throw an error if async: false is sent with parse options when using an async extension.

This allows us to overload the parse method when the async option is set.

const html: Promise<string> = marked.parse(md, { async: true });
const html: string = marked.parse(md, { async: false});
const html: Promise<string> | string = marked.parse(md);

closes #3116

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• If submitting new feature, it has been documented in the appropriate places.

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[UziTech]

@styfle @calculuschild Can I get an approval to merge this and release marked v14