Migrate subtract from if-plugins to builtins

[jmcook1186]

Sub of: #656

What
Move the subtract plugin from if-plugins to if/builtins

Why
This is part of our migration of our core plugins into a standard library in if/builtins.

Context
This is a core feature we want to support, so it is migrating into if.

Prerequisites/resources
no

SoW (scope of work)

• move plugin code into builtins/subtract
• fix any integration issues caused by the migration
• migrate and fix unit tests
• update README documentation updated

Acceptance criteria

• plugin code is migrated to if-builtins
  Given (Setup): The migration has been completed.
  When (Action): A user or developer navigates to if
  Then (Assertion): they will find a folder if/builtins/subtract containing the plugin source code and a README.

• plugin code and unit tests pass and maintain 100% coverage
  Given (Setup): The migration has been completed and a user has downloaded if.
  When (Action): they run npx jest --coverage
  Then (Assertion): they will receive a report showing 100% coverage and 100% passing