Make colorscheme.ts a module, instead of a classic script

In https://github.com/authgear/authgear-server/issues/2058#issuecomment-1148203594 we worked around FOUC by making colorscheme.ts a classic script. But the logic there is now needed to be reused. A classic script cannot import code for reuse. This issue is about

- Reuse the logic in colorscheme.ts, by making it a module.
- Make sure there is no FOUC with this change

Start by locating colorscheme.ts and read the workaround discussed in issue #2058, especially the referenced comment. Refactor it into a reusable module while preserving its current startup behavior, then verify that the change still prevents flash of unstyled content (FOUC).

Written by the indexing model from the issue text.