feat(sqlite): add SQLite writer for converting PostgreSQL schemas

- Implement SQLite DDL writer to convert PostgreSQL schemas to SQLite-compatible SQL statements.
- Include automatic schema flattening, type mapping, auto-increment detection, and function translation.
- Add templates for creating tables, indexes, unique constraints, check constraints, and foreign keys.
- Implement tests for writer functionality and data type mapping.

pkg/writers/sqlite/templates/create_unique_constraint.tmpl

@@ -0,0 +1 @@
+CREATE UNIQUE INDEX {{quote_ident (format_constraint_name .Schema .Table .Name)}} ON {{quote_ident (qualified_table_name .Schema .Table)}} ({{range $i, $col := .Columns}}{{if $i}}, {{end}}{{quote_ident $col}}{{end}});