[plt-scheme] on-execute method in drscheme:language:language<%>

I don't agree this is a programming environment issue. What has worked
best in the past is when the programming environment is a "skin" for
the program (taking into account knowledge of the programming language
too).

In other words, you want to be able to make the choice about
proving/type-checking/whatevering independently of whether or not
you're running in the programming environment, so you don't want to
code that in to any drscheme-specific method.

(Of course, you may wish to have some flag somewhere that some
environment button sets, but I think that's a separate issue.)

Robby