with-reasonml example: better setup

[rase-]

With this setup we're compiling the .re/.ml files directly to source directories with a .bs.js extension, and enabling next.js to pick up that extension for pages.

This has multiple advantages:

• We can re-enable the bs watcher to hot reload components and pages in dev mode when the .re/.ml files are changed
• Use this setup to include ReasonML to existing next.js projects, since pages and components can be either JS or ReasonML, and they will both work with this setup
• Doesn't rely on bs-loader which is deprecated, and follows the officially recommended setup with in-source

[timneutkens]

Nice!