Add an API key config dialogue

Given an installed version of this addon, the user should be able to configure the API key, using an options dialogue in the browser.

This makes it easy for nontechnical users to test. And it's a prerequisite for release on browser webstores.

One solution would be to adapt popup.js/popup.html from GPT-prompter:

https://github.com/giosilvi/GPT-Prompter/blob/master/popup.js https://github.com/giosilvi/GPT-Prompter/blob/master/popup.html

Pros: Easy to adapt; already works

Cons:

• GPT-Prompter code is somewhat messy
• Would require copying GPT-prompter's MIT license into the project*

* For example, a "licenses" folder could contain a file "GPT-Prompter.MIT.License"