fix #7713 feat(nimbus): Add targeting for first run

[yashika-khurana]

Because

• We want to add a new targeting config for mobile first run that uses the boolean property that Fenix and Firefox iOS expose that is true on the first session, and then set both sticky and first run options on the targeting config to true.

This commit

• Adds a new first run targeting for mobile fixes #7713

[eliserichards]

we have this on Fenix. I'm not sure how to do it on iOS 🤔

[jhugman]

• isFirstRun in Fenix
• isFirstRun in Firefox for iOS

The one gotcha here is that for historical reasons, isFirstRun is a string, not a boolean; the jexl expression will be

isFirstRun == 'true'

[jaredlockhart]

Thanks @jhugman ! @yashika-khurana Can you update this to use that expression for the relevant apps pls 🙏

[yashika-khurana]

checks are not happy, let me fixed that

[yashika-khurana]

@jaredlockhart @jhugman thank you for the feedback 🎉