[WIP-01] [Project Solar / Phase 1 / Tokens Pipeline] Main feature branch for $modes support in tokens pipeline

[didoo]

📌 Summary

This is the feature branch for the work needed to add support in the design the tokens pipeline for:

• definition of co-located $modes token values in the src JSON files
• generation of multiple "themed" CSS files in output

It is based on top of #3237

Note: later we may decide to drop this PR and point #3239 directly to #3237 as its base branch

🛠️ Detailed description

This PR will include:

• [WIP-02] [Project Solar / Phase 1 / Tokens Pipeline] Add $modes support to tokens pipeline #3239

🔗 External links

Jira ticket (Epic): https://hashicorp.atlassian.net/browse/HDS-5193

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👀 Component checklist

• Percy was checked for any visual regression
• A changelog entry was added via Changesets if needed (see templates here)

💬 Please consider using conventional comments when reviewing this PR.

📋 PCI review checklist

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've worked with GRC to document the impact of any changes to security controls.
  Examples of changes to controls include access controls, encryption, logging, etc.
• If applicable, I've worked with GRC to ensure compliance due to a significant change to the in-scope PCI environment.
  Examples include changes to operating systems, ports, protocols, services, cryptography-related components, PII processing code, etc.

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