Support configuring a default instant validation level (#93301)
Adds `experimental.instantInsights.validationLevel` so an app can opt
into instant validation for all routes by default.
There are two public levels
* `manual-warning`: currently the default. this mode won't enable
instant validation except where you add `export const unstable_instant =
true` into your Layouts, Pages, and Default files
* `warning`: This mode will enable instant validation for all Page and
Default files. You can override this with `export const unstable_instant
= false` in Layouts, Pages, and Default files to tune what navigations
are validated.
In addition this change disables build time validation.
There is a now undocumented way to get back to running build validation
with
* `experimental-error`
* `experimental-manual-error`
The build validation isn't ready for users because we need to overhaul
the setup where you provide cookies and other sample values so we can
run validation without a live session like we do in dev.
To further support this the two remaining object properties that are
also not ready for active use, `samples` and `from` are being updated to
`unstable_samples` and `unstable_from` respectively.
We will continue to iterate on build time validation but may remove
these APIs or alter their semantics as we figure out a more ergonomic
way to support this setup.
In anticipation of multiple levels the object form of `instant` can be
provided a level value `warning` or `experimental-error`. this latter
option is similarly undocumented so documented behavior will only cover
`warning` which is generally a synonym of true unless you are activating
error by default
