Migrate away from the embedded Express server, embrace pure Vite plugins implementation

[JulianCataldo]

This should not be too difficult, but that will make Gracile a bit less personalized regarding dev/build lifecycle.

Some of the benefits are :

• User can "augment" an existing Vite project with Gracile
• It's becoming a pattern, since Remix opted for this approach
• That will shrink deps. by a good bit. Express is definitely too heavy with a deep dep-tree.
• Internal sources will be fairly simplified.
• Some CLI entrypoint will become obsolete
• Tests should be more stable. For now, Gracile runtime can be fiddly to start/stop, especially when a crash occurs (stuck)

Custom CLI entrypoint will have to be preserved for launching the Vite Runtime when using the server output with user provided server instance (loadable from a file entrypoint option, still undocumented).