fix(typings): improve typing for server_default

fix(typings): improve typing for server_default

Fixed typing issue where the :paramref:`.AlterColumnOp.server_default` and
:paramref:`.AlterColumnOp.existing_server_default` parameters failed to
accommodate common SQLAlchemy SQL constructs such as ``null()`` and
``text()``.   Pull request courtesy Sebastian Kreft.

this sets up a standard type for the server default argument using
an alias, and adds modifications to write_pyi for extremely basic
ability to render type aliases (with limitations).

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #1669
Closes: #1670
Pull-request: https://github.com/sqlalchemy/alembic/pull/1670
Pull-request-sha: e6464647b6e33e077e7baf4bbc5c7549ab570a06

Change-Id: Id25bf7fd706f91aa637adf9b67f0529f1d7d1080