chore: add single-file, multi-file schema example

Schema comes from https://github.com/pgschema/pgschema/tree/main/testdata/include

Author: tianzhou

multifile/.DS_Store

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+# Multi-file Schema Dump
+
+Each file represents a different database object. Different database object types are organized into different directories. The entire directory represents
+the whole schema. The dump is generated by:
+
+```bash
+pgschema dump --host localhost --user postgres --password testpwd1 --db employee --multi-file --file main.sql
+```
+
+`main.sql` is the entry file that uses `psql` compatible `\i` directive to include other files.

multifile/README.md

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+--
+-- Name: email_address; Type: DOMAIN; Schema: -; Owner: -
+--
+
+CREATE DOMAIN email_address AS text
+  CONSTRAINT email_address_check CHECK (VALUE ~~ '%@%');

multifile/domains/email_address.sql

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+--
+-- Name: positive_integer; Type: DOMAIN; Schema: -; Owner: -
+--
+
+CREATE DOMAIN positive_integer AS integer
+  CONSTRAINT positive_integer_check CHECK (VALUE > 0);

multifile/domains/positive_integer.sql

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+--
+-- Name: get_order_count; Type: FUNCTION; Schema: -; Owner: -
+--
+
+CREATE OR REPLACE FUNCTION get_order_count(
+    user_id_param integer
+)
+RETURNS integer
+LANGUAGE sql
+SECURITY INVOKER
+VOLATILE
+AS $$
+    SELECT COUNT(*) FROM orders WHERE user_id = user_id_param;
+$$;

multifile/functions/get_order_count.sql

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+--
+-- Name: get_user_count; Type: FUNCTION; Schema: -; Owner: -
+--
+
+CREATE OR REPLACE FUNCTION get_user_count()
+RETURNS integer
+LANGUAGE sql
+SECURITY INVOKER
+VOLATILE
+AS $$
+    SELECT COUNT(*) FROM users;
+$$;

multifile/functions/get_user_count.sql

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+--
+-- Name: update_timestamp; Type: FUNCTION; Schema: -; Owner: -
+--
+
+CREATE OR REPLACE FUNCTION update_timestamp()
+RETURNS trigger
+LANGUAGE plpgsql
+SECURITY INVOKER
+STABLE
+AS $$
+BEGIN
+    NEW.updated_at = NOW();
+    RETURN NEW;
+END;
+$$;

multifile/functions/update_timestamp.sql

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+-- Main schema file demonstrating \i include functionality
+-- This represents a modular approach to organizing database schema
+-- Includes ALL supported PostgreSQL database objects
+
+-- Include custom types first (dependencies for tables)
+\i types/user_status.sql
+\i types/order_status.sql
+\i types/address.sql
+
+-- Include domain types (constrained base types)
+\i domains/email_address.sql
+\i domains/positive_integer.sql
+
+-- Include sequences (may be used by tables)  
+\i sequences/global_id_seq.sql
+\i sequences/order_number_seq.sql
+
+-- Include trigger function (needed by users table trigger)
+\i functions/update_timestamp.sql
+
+-- Include core tables (with their constraints, indexes, and policies)
+\i tables/users.sql
+\i tables/orders.sql
+
+-- Include other functions (after tables that they reference)
+\i functions/get_user_count.sql
+\i functions/get_order_count.sql
+
+-- Include procedures
+\i procedures/cleanup_orders.sql
+\i procedures/update_status.sql
+
+-- Include views (depend on tables and functions)
+\i views/user_summary.sql
+\i views/order_details.sql

multifile/main.sql

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+--
+-- Name: cleanup_orders; Type: PROCEDURE; Schema: -; Owner: -
+--
+
+CREATE OR REPLACE PROCEDURE cleanup_orders()
+LANGUAGE sql
+AS $$
+    DELETE FROM orders WHERE status = 'completed';
+$$;

multifile/procedures/cleanup_orders.sql

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+--
+-- Name: update_status; Type: PROCEDURE; Schema: -; Owner: -
+--
+
+CREATE OR REPLACE PROCEDURE update_status(
+    user_id_param integer,
+    new_status text
+)
+LANGUAGE sql
+AS $$
+    UPDATE orders SET status = new_status WHERE user_id = user_id_param;
+$$;