In order to prevent problems like #4857 in the future let's move to scoped packages on npm. I don't consider this a breaking change as it needs a manual action to update anyway.

Plan:

• Start publishing beta builds to @xterm/xterm, @xterm/addon-search, etc.
• Publish 5.4 to new scope
• Deprecate old packages with a message redirecting users to the new package name
• update all docs snippets including paths in <script> elements here and on xtermjs.org (added by @jerch)