Refactor edit.js

[shu8]

It might be a bit easier to complete https://github.com/webaverse/xrpackage/issues/69 after separating out some of the core parts of logic from the edit.js file.

This PR just moves chunks of code out into their own modules to make it easier to read and follow, for example:

• Move DOM element definitions into own file
• Move constants and utilities into own file
• Move some world logic into own file
• Move UI classlist handlers into own file
• Move UI drag handlers into own file

I think more logic could be separated but this is a first pass attempt and more things might become obvious during the removal of run.html

[shu8]

I've rebased with master and gone through the latest edit.js manually to make sure none of the recent changes have been lost.

[shu8]

Closing slate PR