invalidateTemplates utility

[pi0]

Context: https://github.com/nuxt/framework/pull/2171/files#r758313330

Add a shortcut to trigger Nuxt builder rebuilding templates.

For Nuxt 3, builder:generateApp should be called.

For bridge compatibility, we can call generateRoutesAndFiles directly.

In the future, we might find better chance for selective template invalidation that needs fine control over vfs plugins for cache support.

Naming

I thought would be safer if we don't include "generate" and "app" since even tough technically we use this stage, it might not be always the case in the future nor clear for users of purpose.

However since this util is active (immediately regenerates) maybe we should call it generateTemplates() or updateTemplates() ?