Add option to use syncronous processor

[The1029]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)

Problem

remark-retext currently returns an async processor with no option to change this.

Solution

By adding an option to request a syncronous processor instead, the plugin would become more versatile.

Alternatives

Do not use remark-retext in syncronous functions.

[JounQin]

You didn't provide any valuable input, why it's needed, why the async version does not work for you.