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)
Problem: Vim9: error for using a string in a condition is confusing.
Solution: Give a more specific error. Also adjust the compile time type
checking for || and &&.
Problem: Vim9: parsing type does not always work.
Solution: Handle func type without return value. Test more closures.
Fix type check offset. Fix garbage collection.
