Modernize and clean up code (attempt 2)

[KonSola5]

This PR:

• formatted every script using Prettier,
• bundles with Prettier config enforcing 2-wide tabs and 120-character code wrap width,
• replaces innerHTML with textContent or innerText whenever innerHTML was used just to add text,
• replaces deprecated keyCodes with codes whenever the key is 100% handled by JstrisPlus (changed the config field names from "_KEYCODE" to "_KEY" along the way),
• changes bracket notation to dot notation whenever possible,
• changes string concatenation to template literals whenever possible.

Known bug (which was present before, now it's modified): When refreshing the tab, "Enable chat timestamps" and "Take a screenshot with this button" keys are visually empty, but they still work. (Config() for some reason is not initialized at the moment when keys are filled into the textfields).

Attempt 2, because the forked repository kept adding changes I did not want here (such as modified license or a submodule), now it's done using a separate local git repository that only handles the jstris-plus branch.