Add public topspace-height function for use from external packages to allow them integrate more easily with topspace.

• Returns the top space line height in the selected window
• May be useful for example by any package trying to determine the distance in lines between the current line and top of the window (which is no longer guaranteed to be at window-start when using topspace).

Checklist

• I have read the topspace contributing guidelines
• My changes follow the Emacs Lisp conventions and the Emacs Lisp Style Guide
• I've used the latest version of package-lint to check for packaging issues, and addressed its feedback
• The new code is not generating bytecode warnings
• I've updated the readme (if adding/changing user-visible functionality)
• I have confirmed some of these without doing them