Enable custom themes to theme Compound

[robintown]

This adds a simple compound section to custom themes that enables any of Compound's semantic tokens to be overriden, like so:

{
  ...
  "setting_defaults": {
    "custom_themes": [
      {
        "name": "My Theme",
        "is_dark": false,
        "fonts": {
          ...
        },
        "colors": {
          ...
        },
        "compound": {
          "--cpd-color-icon-accent-tertiary": "var(--cpd-color-blue-800)",
          "--cpd-color-text-action-accent": "var(--cpd-color-blue-900)"
        }
      }
    ]
  }
}

Closes element-hq/element-web#26285
Requires element-hq/element-web#26993
Requires element-hq/element-web#27004

Notes: Enable custom themes to theme Compound, and remove the previous username-colors and avatar-background-colors options. To customize user/avatar colors, you must instead override the --cpd-color-text-decorative- and --cpd-color-bg-decorative- design tokens provided by Compound. See the documentation.

---

Here's what your changelog entry will look like:

🦖 Deprecations

• Enable custom themes to theme Compound, and remove the previous username-colors and avatar-background-colors options. To customize user/avatar colors, you must instead override the --cpd-color-text-decorative- and --cpd-color-bg-decorative- design tokens [provided by Compound](https (#12240). Fixes Respect theme avatar-background-colors element-hq/element-web#26285.

[t3chguy]

This isn't a real SDK so we don't maintain backwards compatibility nor full semver, worth raising as a deprecation so its higher in the changelog though

[robintown]

Ah thanks, I didn't realize we had a deprecation option for changelogs

[PeterKleinTele2]

This is great news. Thanks to all involved!