[FEATURE]: Add IF NOT EXISTS when adding new values to existing POSTGRES custom types

### Describe what you want

When we add new values to existing POSTGRES custom type DrizzleORM generates SQL like below:

`ALTER TYPE "custom_type" ADD VALUE "type3"`

But it will be safer if generates SQL like below:

`ALTER TYPE "custom_type" ADD VALUE IF NOT EXISTS "type3"`

The issue names no files or tests. Start by locating the PostgreSQL migration SQL generation for adding values to existing custom types and compare its current statement with the requested one. Done means generated SQL uses IF NOT EXISTS and the relevant migration tests pass.

Written by the indexing model from the issue text.