Fix watch mode when using presets

[mvestergaard]

This is a fairly naive attempt at fixing watch mode while using a preset.

The problem stems from the key of the configuration using the preset pointing to a folder, and in the end being ignored by chokidar.

The easy fix would be to just leave out any generates configuration that uses a prefix, but I've tried to make it do the right thing, by combining the key (assuming it's a path) with the configured extension (if one exists)

For config

some/folder:
  preset: near-operation-file
  presetConfig:
      extension: .generated.tsx

You should end up with an ignore of some/folder/**/*.generated.tsx

Related #2069

[dotansimha]

Great catch, thank you so much, @mvestergaard !