[5.x] Add firstOrfail, firstOr, sole and exists methods to base query builder

[duncanmcclean]

This pull request adds various useful methods to Statamic's base query builder:

• firstOrFail
  • Gets the first result if one exists, otherwise, it throws an exception.
• firstOr
  • Gets the first result if one exists, otherwise, it executes the provided callback.
• sole
  • Gets the first & only item if one exists. It'll throw an exception if no results can be found or if more than 1 result is found.
• exists
  • Returns true if the query returns one or more results.

On the back of #9815, I was thinking a few of Laravel's first/find methods could be added to our "base" query builder since they're useful for everything, not just entries & terms.