Expose environment function should_quit

[hpdeifel]

This is useful to check if the user pressed C-g during a long running computation. From the documentation:

If your module includes potentially long-running code, it is a good
idea to check from time to time in that code whether the user wants to
quit, e.g., by typing ‘C-g’ (*note Quitting::). The following function,
which is available since Emacs 26.1, is provided for that purpose.

-- Function: bool should_quit (emacs_env *ENV)
This function returns ‘true’ if the user wants to quit. In that
case, we recommend that your module function aborts any on-going
processing and returns as soon as possible.

Notice that this is only available since Emacs 26.1, so some runtime check for the Emacs version would probably be needed.