Project Solar: update format of the tokens to use the DTCG format

[didoo]

📌 Summary

This PR builds on top of #3139 to change the format used for our design tokens to follow as much as possible the DTCG format.

We will leverage this new format to introduce the “experimental” $modes object for multi-theme (colocated theme) tokens.

🛠️ Detailed description

In this PR I have:

• converted src tokens to DTCG format
  • initially I added new files in separate folders, for development, and when everything was working I replaced the old files with the new ones
• updated main build script so that it would support the new format
  • in the process I have made some changes to the code, at times to bypass some limitations or strange handling of the DTCG format (still in the early stages), in others to simplify/modernize how we process the tokens, and in some cases fixed some logic that didn't work (the throw assertions) but we didn't notice
• updated the scripts used to generate the CSS helpers to support the new format
• run pnpm build to make sure the output in dist remained exactly the same
  • except for a couple of JSON files, that were updated because of the new tokens format (see commit comment)
• updated the website logic for the "foundations/tokens" page, to use the new format of the JSON files

Outcome: apart from the JSON files, all the CSS files have remained exactly the same, which was the goal from the beginning: this gives us confidence that we're not introducing

Website preview: https://hds-website-git-project-solar-phase-1hds-5200s-f936f2-hashicorp.vercel.app/foundations/tokens

🔗 External links

Jira ticket: https://hashicorp.atlassian.net/browse/HDS-5462

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👀 Component checklist

• Percy was checked for any visual regression

💬 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.

[vercel]

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Project	Deployment	Preview	Updated (UTC)
hds-showcase	Ready	Preview	Sep 5, 2025 7:30pm
hds-website	Ready	Preview	Sep 5, 2025 7:30pm

[KristinLBradley]

Is the value supposed to include the unit as well?

    "$value": "3px",
    "unit": "px",

It shouldn't be formatted as in this example?
https://www.designtokens.org/tr/drafts/format/#example-2
https://www.designtokens.org/tr/drafts/format/#dimension

    "$value": {
      "value": 3,
      "unit": "px"
    },