Make type generation with svelte-kit package configurable

[janosh]

Is your feature request related to a problem? Please describe.
Types are great and the vast majority of packages should have them but life has many degrees of freedom so an escape hatch down the line can help with unforeseen circumstances.

Describe the solution you'd like
Add boolean flag config.kit.package.emitTypes.

Describe alternatives you've considered
N/A

How important is this feature to you?
Medium

Additional context
Related to #1646. See the discussion following this comment.