Add Supabase postgrestools to extensions.json

[hbmartin]

• 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 introduces adds Supabase postgrestools to extensions.json so that I can install it on OSS forks.

I have opened an issue on the upstream repo requesting direct publishing: supabase-community/postgres-language-server-vscode#19

[hbmartin]

I have signed the ECA

[tadeumaia]

This would be a great addition to the community.