Provide a way to detect if a preview session is active

[angeloashmore]

Is your feature request related to a problem? Please describe.

The only way to know if a preview session is active today is to manually check for Prismic's preview cookie. Even then, only checking for presence of the cookie is not sufficient; it must be parsed to determine if a session is active.

Knowing if a preview is active is useful for displaying preview-only UI, such as a banner letting a content writer know that they are previewing draft content.

Describe the solution you'd like

The client could provide a method that checks if a preview session is active. The client is scoped to a repository name, so the returned data could be specific to a repository. That level of specificity may be useful in sites that source content from multiple repositories.

The preview cookie is already automatically detected in resolvePreviewURL(), which takes into account the global scope (i.e. document) or a server request. The same logic could be used to support most runtimes.

Such a method could look like this:

client.isPreviewActive() // => true or false

Describe alternatives you've considered

Load the cookie, parse it, check if a specific repository name is present, return true or false. Very procedural and requires specific knowledge of Prismic's preview implementation.

Additional context

Feature requested by @samlfair