Extension cleanup

[EricBAndrews]

This PR cleans up the extensions folder, making the following changes:

• All extensions are now named Base+Extension
• All extension files now contain just one extension or variable. Exceptions are made for extensions that group static members (e.g., Color+Colors) and extensions that define the same function in multiple contexts (e.g., Error+ToEquatableError)
• All extensions are now grouped in folders according to what they extend, with some exceptions:
  • Mocks all live in the Mock group
  • View modifiers all live in the View Modifiers group
  • Tracker item conformance extensions all live in the Tracker Items group
• Removed some unused extensions

[boscojwho]

Improvement over current state:
It builds, it runs, it heckin` passes 🐈