'Invalid exports' in vite-plugin-vanilla-extract when loading external library containing style files with file extensions `.css.ts`

Describe the bug

vite-plugin-vanilla-extract in an application context in combination with a "third-party" library using web-components with LitElement throws the invalid exports error when the component loads its own styles from an separate file, named styles.css.ts which clashes with the .css.ts files from vanilla-extract.

[vite-plugin-vanilla-extract] Invalid exports.

You can only export plain objects, arrays, strings, numbers and null/undefined.

Creating a style file containing the styles for the web-component called styles.css.ts within the component library for the web-component in MyCustomElement.ts will produce the issue in the application.

Adding an additional option for the vanillaExtractPlugin which offers to set the file extension which is used for files which contains "vanilla-extract" content would most likely solve this issue.

vanillaExtractPlugin({
  fileExtension: '.css.ts',
});

Reproduction

https://github.com/tkgroot/vanilla-extract-issue/tree/master

System Info

System:
    OS: macOS 13.6.4
    CPU: (10) arm64 Apple M1 Max
    Memory: 127.14 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.5.1 - ~/.nvm/versions/node/v22.5.1/bin/node
    npm: 10.9.0 - ~/.nvm/versions/node/v22.5.1/bin/npm
    pnpm: 10.4.1 - ~/.nvm/versions/node/v22.5.1/bin/pnpm
  Browsers:
    Chrome: 132.0.6834.110
    Safari: 17.5

Used Package Manager

npm

Logs

Validations

• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• The provided reproduction is a minimal reproducible example of the bug.