Fix mode generation for plugin-sass + plugin-css

[drwpow]

Changes

• Fixes a bug outlined in Mode output in plugin-sass (CSS variable mode) uses the tokens own css-variable for all modes, not their respective targets #199 where plugin-sass uses the wrong aliases inside $extensions.mode
• Fixes a CLI bug where if tokens.json is missing, it prints out a weird error rather than “Can’t find tokens.json”
• In plugin-sass, the default key was replaced with "." to prevent conflicts with users have a mode named "default". The problem doesn’t exist in other outputs like plugin-js.
• (Chore) Shopify’s tokens.json was accidentally deleted; this just puts it back

How to Review

• Tests updated; tests should pass

[changeset-bot]

🦋 Changeset detected

Latest commit: 8b080b2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@cobalt-ui/cli	Patch
@cobalt-ui/plugin-sass	Minor

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