Move previewer assets under a version segment

Unity's .loader.js, .framework.js and .wasm are built as a set and are not
interchangeable across versions. A flat path would hand the same build to
every consumer regardless of which previewer they expect, so the version
goes in the URL: Unity/previewer/0.20.0/Build/Previewer.*

These assets are v0.20.0 -- @reactunity/previewer stamps its version into
index.html's <title> at publish time, which is what identifies them.

@reactunity/scripts serves a previewer page pointing at this path, so a new
previewer build means a new directory here plus a bumped URL there. Nothing
overwrites an existing version.

index.html is kept alongside so this URL is also a working standalone
previewer -- its asset references are relative.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: gkurt