Add rocketseat.theme-omni

[thejoaov]

• 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 rocketseat.theme-omni on the extensions list.

[filiptronicek]

Hey there @thejoaov, since there hasn't been any commit activity for the past 4 years in the origin repo, I took the liberty to one-time publish it using our service account, so that we don't have to maintain the entry in the extension list.

You can find it here: https://open-vsx.org/extension/rocketseat/theme-omni