Adds up- and downgrade scripts for the support for type parameter
defaults.
In the upgrade direction we do nothing, matching the behaviour of
`getDefault` not having a result for old databases.
In the downgrade direction, we explicitly remove the relevant child (via
the `py_exprs` database relation) for `TypeVar`, `TypeVarTuple`, and
`ParamSpec` parameters.
In the upgrade direction, we simply do nothing.
In the downgrade direction, we remove the two new relations, and
also any `Stmt` nodes corresponding to `TypeAlias` nodes.
