Added ExifTags enums

[radarhere]

Alternative to #6586

This PR effectively copies the data from ExifTags.TAGS and ExifTags.GPSTAGS into two new IntEnum classes, ExifTags.Base and ExifTags.GPS.

The advantage of this is ease of mapping a tag name to a tag number. Rather than

>>> from PIL.ExifTags import TAGS
>>> for value, name in TAGS.items():
...   if name == "ImageDescription":
...     print(value)
... 
270

users could

>>> from PIL.ExifTags import Base
>>> Base.ImageDescription.value
270