patch 8.2.2188: Vim9: crash when calling global function from :def function

patch 8.2.2188: Vim9: crash when calling global function from :def function

Problem:    Vim9: crash when calling global function from :def function.
Solution:   Set the outer context.  Define the partial for the context on the
            original function. Use a refcount to keep track of which ufunc is
            using a dfunc. (closes #7525)