Allow users to edit scheme file temporary on popup page [Hacktoberfest]

[FoxRefire]

Minimum requirement

1. When user clicks Edit button after drop-down scheme selector element, modal opens up.
2. In the opened modal, should place two elements: textarea and OK button.
3. The value of textarea is set by default to the scheme code selected by the user.
4. When the user clicks the Guess button, instead of retrieving the code directly, expects the string retrieved from textarea to be interpreted as a scheme file and executed.

Implementation hints

The scheme retrieval process is performed in popup/main.js#L23.

Add an element that satisfies the requirements described in the specification to popup/main.html and make sure that the scheme code is obtained from textarea.

On change event happens at drop-down element, update textarea's value