Fix #5859: Fix Security Tokens issue

[Brandon-T]

Security Review

• https://github.com/brave/security/issues/984

Summary of Changes

• Refactor BraveSearch & BraveTalk to use WebKit's promises API, and to use the messageToken properly.
• Refactor scripts attaching to window to be anonymous if they can be.
• Refactor all public facing APIs to not use tokens directly, and to call a function to post messages (this function being embedded in an anonymous function, or not page facing).

This pull request fixes #5859

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()
• New or updated UI has been tested across:
  • Light & dark mode
  • Different size classes (iPhone, landscape, iPad)
  • Different dynamic type sizes

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).

[thypon]

pending dynamic test