feat: :alien: implement readwise throttling

[johannrichard]

Due to cross-origin / request limitations, access to the "Retry-After" header is limited and when Readwise returns status 429, the plugin tries to throttle with a NaN. However, the Readwise API returns the throttling info in the body. This change implements the necessary parsing and catch, with the default 20 req./min throttling in case parsing wouldn't work.

(Now working with the correct changes.)

[johannrichard]

See the following article for an explainer of what I presume is the source of the original problem

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Expose-Headers
• https://fetch.spec.whatwg.org/#cors-safelisted-response-header-name

[jsonMartin]

Hey @johannrichard , I just saw your open pull requests - I can't get to them just yet, but will review them when I have some spare time.

Thank you for your hard work on these PRs!

[jsonMartin]

LGTM 👍🏼