Change `__pattern_library_view` to `is_pattern_library` and make it an official "API"

[zerolab]

Is your proposal related to a problem?

There are cases when it comes handy to check whether a pattern is displayed in the pattern library or is an actual page.

We have __pattern_library_view in the context, however you cannot use it like {% if __pattern_library_view %} because TemplateSyntaxError: Variables and attributes may not begin with underscores: '__pattern_library_view'

Describe the solution you'd like

Change __pattern_library_view to is_pattern_library in the context

Describe alternatives you've considered

My workaround was to add a dummy variable in the yaml and use that