Fix duplicate semicolon in built CSS styles

[simonporter007]

WHY are these changes introduced?

Fixes #13151

Came across this issue when trying to add tailwind v4 to a Shopify app template. Tailwind has switched CSS parsers in v4, and the vite plugin now errors if the app imports the Polaris styles with the error:

[plugin:@tailwindcss/vite:generate:serve] Unexpected semicolon
.../node_modules/.pnpm/@[email protected][email protected][email protected][email protected]/node_modules/@shopify/polaris/build/esm/styles.css

WHAT is this pull request doing?

This PR removes the semi-colons from the shopify-react mixins which cause the double semicolons in the final built CSS file. It also removes the semicolon from the shadow button primary variable from within polaris-tokens, which was causing an additional duplicate semicolon.

How to 🎩

• Storybook styling should remain unaffected
• Using the tailwindcss v4 parser should not throw errors

To test with tailwind, you should:

• Go get the latest release of tailwind on GitHub at https://github.com/tailwindlabs/tailwindcss/releases/ and find the appropriate one for your operating system. Make sure it is executable if necessary.
• Write a small input.css file that imports the shopify polaris built CSS file:

@import './build/esm/styles.css';

• Parse the file with ./tailwindcss --input input.css --output output.css

N.B. It should be possible to recreate the error by parsing the styles.css before these changes were made (or in a separate existing project that uses shopify polaris):

@import './node_modules/@shopify/polaris/build/esm/styles.css';

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

🎩 checklist

• Tested a snapshot
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide