Add benich-theme extension

[lucastomic]

Benich theme is a dark theme

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @lucastomic on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[zed-industries-bot]

	Warnings
⚠️	This PR doesn't include changes to extensions.toml. If you are creating a new extension, add a new entry to it. If you are updating an existing extension, update the version number in the corresponding entry.

Generated by 🚫 dangerJS against 8a96fec

[notpeter]

Hi!

A couple things:

1. You need to sign the CLA and follow the instructions in the comment above to recheck the PR
2. You need to add your theme/version to extensions.toml (this repo)
3. You need to sort .gitmodules, either manually or with pnpm run sort-extensions (this repo)
4. 2. Your theme json needs to be in a subdirectory called themes (your repo)