Add "db-schema-visualizer"

[wallzero]

add https://github.com/BOCOVO/db-schema-visualizer

• I have read the note above about PRs contributing or fixing extensions
• I have tried reaching out to the extension maintainers about publishing this extension to Open VSX (if not, please create an issue in the extension's repo using this template).
• This extension has an OSI-approved OSS license (we don't accept proprietary extensions in this repository)

Description

This PR adds the db-schema-visualizer extension from VSCode.

Existing issue from June 25th