Name changes and read-only wrappers for consistency

[Vectorized]

Some changes to make the naming scheme consistent,
and make certain variables private with internal view getters for read-only access.

contracts/ERC721A.sol

• Variable _currentIndex made private.

  Created _nextTokenId() internal view function to return the value instead.

• Variable _burnCounter made private.

  Created _totalBurned() internal view function to return the value instead.

These variables are not intended for direct writing by deriving contracts.

The read-only functions are more consistent in naming with existing functions like _totalMinted() and _startTokenId().

This also simplifies accessing the values with Diamond upgradeables
(users can simply call _totalBurned() instead of ERC721AUpgradable.storage()._burnCounter).

contracts/extensions/ERC721AOwnersExplicit.sol

• Error QuantityMustBeNonZero renamed to InitializeZeroQuantity.

  More consistent naming with other custom errors, such as MintZeroQuantity.

• Error AllOwnershipsHaveBeenSet renamed to AllOwnershipsInitialized.

• Variable nextOwnerToExplicitlySet renamed to _currentIndexOwnersExplicit and made private.

  Created _nextTokenIdOwnersExplicit() internal view function to return the value instead.

• Function _setOwnersExplicit renamed to _initializeOwnersExplicit.

  More consistent naming with function _initializeOwnershipAt.

Similarly, the variable is not intended for direct writing by deriving contracts.

Semantically, "initialize" is more appropriate than "set" for their intended use cases,
as users cannot decide on the values to be written.