Support Informix trailing constraint names and share constraint rendering

[minleejae]

Informix places an optional constraint name after its definition, for example:

ALTER TABLE child ADD CONSTRAINT FOREIGN KEY (id)
REFERENCES parent(id) CONSTRAINT fk_child;

Support this form for primary, unique, foreign and check constraints, including unnamed constraints. The existing constraint models expose the name, columns, referenced table and CHECK expression; NamedConstraint.ConstraintNamePosition preserves the name's placement. Shared prefix/suffix rendering keeps model and deparser output consistent while preserving the existing leading-name API defaults.

Validation:

• Full Gradle check and Maven verify.
• Original primary/foreign/unique reproducers, quoted and omitted names, composite keys, mutable AST names, subsequent actions/statements, and malformed input.
• SQL round-trips, CHECK expression visitor customization, referenced-table traversal, and existing CREATE/ALTER regression tests.
• Syntax checked against the Informix ADD CONSTRAINT documentation.

Fixes #271.