feat(themes): add ionic theme tokens (#30708)

- Updates the token interface to accept a numeric scale.
- Updates the base tokens to use the new interface.
- Adds tokens for the `ionic` theme based on the existing design tokens.
- Excludes certain keys (`enabled`, `rippleEffect`, `formHighlight`) from the generated CSS variables.
- Removes the `--background` and `--ion-background-color` from the global `ionic` theme overrides, allowing it to use the dark palette.
- Removes the different Ionic colors from the `ionic` theme, ensuring all of the themes use the same named (primary, secondary, etc.) colors.
- Updates the testing script to pull in the tokens based on the theme & palette passed to the URL.
- Adds a new npm script, `build.themes`, to generate the theme files.

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Co-authored-by: Brandy Smith <6577830+brandyscarney@users.noreply.github.com>