Feature: Add --no-clobber switch to dbml2sql

The SQL generated from DBML assumes it is going into an empty database. It might be useful to include a `--no-clobber` flag in the command to modify the output to add protective clauses to the SQL.

For example, instead of
```sql
CREATE SCHEMA "mySchema"
```

`--no-clobber` would emit
```sql
CREATE SCHEMA IF NOT EXISTS "mySchema"
```

Some object types require a more comprehensive template than `iF NOT EXISTS`:
```sql
DO $$
BEGIN
CREATE TYPE "mySchema"."email_type" AS ENUM (
'PERSONAL',
'WORK',
'OTHER'
);
EXCEPTION
WHEN duplicate_object THEN NULL;
END $$;
```

```sql
DO $$
BEGIN
IF NOT EXISTS (
SELECT 1
FROM pg_constraint
WHERE conrelid = 'mySchema.email'::regclass
AND contype = 'f'
AND conname IN ('fk_email_contact', 'vip_contact_id_fkey')
) THEN
ALTER TABLE "identity"."email"
ADD CONSTRAINT "fk_email_contact"
FOREIGN KEY ("contact_id") REFERENCES "identity"."customer" ("id") DEFERRABLE INITIALLY IMMEDIATE;
END IF;
END $$;
```

**NOTE:** _In order to protect constraints in this way, DBML would need to name them deterministically so the names could be used in the protective query shown above._