Standalone editor: Remove dependency to DOM utils

[JiuqingSong]

In this change I remove the following dependencies:

• findClosestElementAncestor: Use Element.closest instead
• matchesSelector: Use Element.matches instead
• Browser: Add a Environment object in EditorCore and for now it only has isMac property
• isModifierKey: Create a copy
• isCharacterValue: Create a copy
• readFile: Create a copy
• getMetadata: Use existing API updateImageMetadata instead
• contains: Use Element.contains instead
• parseColor: Create a copy
• createElement: Use document.createElement instead