`symbols()` function

### What would you want to propose?

https://drafts.csswg.org/css-counter-styles/#symbols-function

### Suggested solution

Transpile to `@counter-style`

### Additional context

_No response_

### Validations

- [x] Follow our [Code of Conduct](https://github.com/csstools/postcss-plugins/blob/master/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

### Would you like to open a PR for this feature?

- [x] I'm willing to open a PR

No implementation file, test, or entry point is named in the issue. Start by reading the CSS Counter Styles symbols() specification and locating the PostCSS plugin responsible for related CSS transpilation; then determine how its existing tests establish the expected output, with completion meaning symbols() is transpiled to @counter-style and covered by tests.

Written by the indexing model from the issue text.