Add nord-deep theme

[jxnata]

Adds Nord Deep, a dark theme for Zed based on the Nord palette with a slightly deeper editor background (#1d2129).

Repository: https://github.com/jxnata/zed-theme-nord-deep

Checklist:

• Extension tested locally as a dev extension
• extension.toml includes all required fields (id, name, version,
  schema_version, authors, description, repository)
• MIT license included
• Theme JSON validates against https://zed.dev/schema/themes/v0.2.0.json
• Extension id does not contain "zed" or "extension"
• Repository is publicly accessible

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @jxnata 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'.

[jxnata]

@cla-bot check

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @jxnata 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'.

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[jxnata]

@cla-bot check'

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!