feat: allow linked flag variants to control recording

[pauldambra]

see https://posthoghelp.zendesk.com/agent/tickets/10806

Prepares the ground for a change in posthog allowing people to choose boolean linked flags or a particular variant of a multivariant flag to control recordings

fly-by adds an _isNullish check since typing _isNull(blah) || _isUndefined(blah) was tiring me out

[github-actions]

Size Change: +163 B (0%)

Total Size: 860 kB

Filename	Size	Change
dist/array.full.js	185 kB	+38 B (0%)
dist/array.js	126 kB	+40 B (0%)
dist/es.js	126 kB	+40 B (0%)
dist/module.js	126 kB	+40 B (0%)
dist/recorder-v2.js	106 kB	+5 B (0%)

ℹ️ View Unchanged

Filename	Size
dist/exception-autocapture.js	12 kB
dist/recorder.js	58.6 kB
dist/surveys-module-previews.js	62 kB
dist/surveys.js	58.3 kB

_{compressed-size-action}