Set up official shinylive.io with dev Shiny for Python and R

We could build a version of shinylive on a nightly cadence that uses

• Production shinylive.io
• posit-dev/py-shiny@main
• rstudio/shiny@main

It'd be especially helpful if users could switch between these versions by adding dev somewhere in the URL, e.g.

• https://dev.shinylive.io/py/editor or
• https://shinylive.io/dev/py/editor or
• https://shinylive.io/py/dev/editor.

This intersects with the Shiny extension for VS Code in two ways:

1. We currently advertise https://posit-dev.github.io/shinylive as the host for "dev" shinylive. We should update the list of hosts here.

2. We currently assume that the language is the second part of the path, but this can easily be adjusted.

Start by reviewing the proposed dev-host options and the referenced shiny-vscode files: package.json lines 194-206 and src/shinylive.ts lines 544-550. Define how nightly builds use the Shiny main branches and how users select them by URL; done means the dev host works and the VS Code host and path assumptions match it.