Add support for PostgreSQL-style `ALTER COLUMN` syntax
PostgreSQL (and standard SQL) uses `ALTER COLUMN` instead of MySQL's `CHANGE COLUMN` / `MODIFY COLUMN` for modifying existing columns. This adds parsing and schema evaluation for the five main sub-commands:
- `ALTER COLUMN col TYPE newtype`
- `ALTER COLUMN col SET NOT NULL`
- `ALTER COLUMN col DROP NOT NULL`
- `ALTER COLUMN col SET DEFAULT expr`
- `ALTER COLUMN col DROP DEFAULT`
These can be combined with other actions in a single `ALTER TABLE` statement, eg.:
```
ALTER TABLE t
DROP COLUMN old_col,
ALTER COLUMN col TYPE SMALLINT,
ALTER COLUMN col SET NOT NULL;
```
Migration generation (inverse actions + SQL fragments) is also supported for all five variants.