Tilde grabbing: Create grab-manager.js

[patriboz]

This branch was branched off https://github.com/webaverse/app/tree/patriboz/tilde-grabbing And follows after https://github.com/webaverse/app/pull/3714

The scope of this PR is to move all functionality for scene editing from game.js to grab-manager.js. Functionality itself wasn't changed. The exported instance of GrabManager is hooked into the main game loop in webaverse.js to receive updates. Key bindings for scene editing are changed accordingly in io-manager.js.

makeHighlightPhysicsMesh was moved to utils.js, since it rather belongs there and is used by game.js and grab-manager.js.