MMPD-1518: Introduce new browsing experience feature flag

[ziad-saab]

Description

Introduces the base code to start developing the changes to the browser shown in this Figma >> https://www.figma.com/design/3TneFPnMo203zPPkVqthJZ/Mobile-Explore-%2F-Browser?node-id=46-318&p=f&t=FBEMjJMtPYZZbHzV-0

Changes:

• Introduce MM_NEW_BROWSING_EXPERIENCE_ENABLED env var
• Create placeholder screen for new browsing experience
• Show placeholder screen instead of current browser when flag is on

NewBrowsingExperience* components and variables will probably be renamed once the feature is rolled out.

Related issues

Fixes: https://consensyssoftware.atlassian.net/browse/MMPD-1517

Manual testing steps

1. Set MM_NEW_BROWSING_EXPERIENCE_ENABLED to "true" in your .js.env
2. Build the app
3. Navigate to the browser view
4. Expect a placeholder view called NewBrowsingExperienceView with text "New Browsing Experience Placeholder"

Screenshots/Recordings

Before

After

Pre-merge author checklist

• I’ve followed MetaMask Contributor Docs and MetaMask Mobile Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[ziad-saab]

I have read the CLA Document and I hereby sign the CLA

[sonarqubecloud]

Quality Gate failed

Failed conditions
27.3% Coverage on New Code (required ≥ 60%)

See analysis details on SonarQube Cloud