Add `DropTable` as DDL statement.

[giant0791]

Add the capability to drop tables via DDL statements:

students = Table('students', ...)
stmt = DropTable(students)
with engine.connect() as connection:
    connection.execute(stmt)

normlite has two representations of the same logical table:

1. The user-facing Notion database (actual data)
2. The system tables row (metadata / catalog)

These two objects are logically inseparable.

Dropping a table must transition both representations into the “dropped” state.

Consider the following rule to enforce:

There must be exactly one place in the codebase that transitions a table into the dropped state.