feat: reuse NOT NULL constraints for PostgreSQL column definitions

[minleejae]

PostgreSQL 18 supports named column NOT NULL constraints and NOT NULL NO INHERIT. JSqlParser already models the table form, but the column form is rejected or loses the connection to that structured constraint model.

Reuse NotNullConstraint for named/non-inheritable column constraints, with an explicit flag for the implicit target column. CREATE TABLE and ALTER TABLE ADD COLUMN share the grammar, including the existing table-level NO INHERIT parser. Plain unnamed NOT NULL retains its existing ColumnOption.Kind.NULLABILITY representation. Traversal does not invent a second column expression for an implicit target.

Validation:

• Full Gradle check with Java 17, including grammar ambiguity and static analysis.
• Tests cover CREATE/ALTER, quoted names, AST mutation, following defaults, legacy nullability, traversal, and parse/deparse round trips.
• Six PostgreSQL 18.6 execution cases compare original, toString(), and deparser SQL and constraint catalogs. Each confirms that the parent rejects NULL while a child does not inherit the NO INHERIT constraint. PostgreSQL and the parser both reject NOT NULL ENFORCED.

Use Dialect.POSTGRESQL, consistently with the existing PostgreSQL constraint paths. Reference: PostgreSQL 18 CREATE TABLE.

[manticore-projects]

Thank you!