refactor: Define semantic border radius LESS vars

[lokesh]

https://github.com/internetarchive/openlibrary/issues/11555

Creates a set of border radius variables that provide a single point of control for updating font families across the app.

A separate PR will update the codebase to utilize the new variables.

Screenshot

No visual changes.

Stakeholders

@cdrini @mekarpeles @jimchamp