Convert system checks for max_length and max_digits to __init__() checks.

[Jacob Walls]

#28120 extended a system check validating CharField.max_length to also validate that booleans are not provided.

However, system checks only check model fields, not fields constructed for use in the output_field argument of an Expression, e.g. when using Cast(). (Invalid inputs to Cast() error out at the db level instead of raising nicely.)

I'm not suggesting to move a bunch of checks over to *Field.__init__(), but I am suggesting that for simple checks on isinstance and > 0 that we could error out in {Char,Decimal}Field.__init__ instead of via a system check, to catch additional usage mistakes.

Another use case besides output_field would be for database setup before tests run checks, as in #34727.