fix: convert breakpoints variables to values instead of strings

[AxelPeter]

Changes description

Change value of Breakpoints design tokens from strings to values to be used in var()

Context

With Web Checkout, we try to rely the most on Vitamin design tokens. But i have an issue when using the breakpoints design tokens. Currently, they are strings and can't be used as value in var()

/* Don't work */
--foo: "900px";
max-width: var(--foo);

/* Works */
--foo: 900px;
max-width: var(--foo);

Checklist

• [x] Make sure you are requesting to pull a topic/feature/bugfix branch. Please, don't request directly from your main!
• [x] Check commits & PR names matches our requested structure. It must follow the https://www.conventionalcommits.org pattern.
• [x] Check your code additions will fail neither code linting checks.
• [x] I have reviewed the submitted code.
• [x] I have tested on related showcases.
• [x] If it includes design changes, please ask for a review design-system-core-team-design GitHub team.

Does this introduce a breaking change?

If these tokens are used as strings somewhere, it could be a breaking change. But any others design tokens are values and not strings. I've checked in the codebase any use as strings but didn't found any, so it should not be a problem here

[CLAassistant]

All committers have signed the CLA.

[lauthieb]

Thanks @AxelPeter for this PR, good catch! I just checked and I think this will not have consequences for our existing consumers. So we can merge :)