feat(export): DBML and PDF export of the diagram #1285

Part of #1286.

Problem

The diagram exports to PNG, SVG and Mermaid. DBML is what people paste into dbdiagram / dbdocs and into documentation, and PDF is what they attach to documents.

Scope

• DBML export of the shown tables: tables, columns with pk, unique, not null, default, increment, note; enums; refs with <, -, >; table groups and sticky notes; header colours.
• PDF export: the diagram as on screen, one page fitted (and an A3 / A4 choice).
• Both from the existing Export menu; DBML also Copy to clipboard.

Acceptance

• The DBML of a sample schema parses in dbdiagram and draws the same tables and refs.
• PDF matches the screen.
• Golden tests for the DBML text.