feat(edge-bundler)!: exclude unrouted functions from the bundle

A function with no route can never be invoked, so bundling it only
eagerly loads code that can never run, which can surface import-time
failures for functions that are not even in use. Behind the
`edge_bundler_exclude_unrouted_functions` feature flag, unrouted
functions are left out of the bundle, and when nothing is left to bundle
we produce neither a bundle nor a manifest.

BREAKING CHANGE: the `manifest` property returned by `bundle` is now
`Manifest | undefined`. It is `undefined` when no function is bundled.