refactor(checkbox): use the new tokens (#29564)

Issue number: internal

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## What is the new behavior?
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this PR. -->

- updated ionic.checkbox.scss with ionic tokens and removed references
to checkbox.scss partial
- removed ionic.checkbox.vars.scss file
- update snapshots resulted from slight differences due to the use of
the updated tokens.
- added theme-ionic html file to test basic use-cases with ionic theme.
- border-radius now won't auto scale with `--size`. This was not
possible to do using the gloabls tokens variables and the css calc(), as
it was before. Besides, this is not something we are very interested to
offer on Ionic theme. It should be on developer to also update the
`--border-radius` variable if they want, after changing `--size`.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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Co-authored-by: Maria Hutt <thetaPC@users.noreply.github.com>