Context overrides should not override functions?

[Skaiir]

Describe the Bug

This is a follow-up to https://github.com/nikku/feelin/issues/62.

Uncertain if this is 100% a bug, but see: https://nikku.github.io/feel-playground/?e=sum%28%5B1%2C2%5D%29&c=%7B%0A++%22sum%22+%3A+20%0A%7D%0A&t=expression&st=true.

Basically, context variables are overriding base functions now following the change we made to allow variables like "date" to be used, but this causes unexpected behaviors for a lot of people in forms who define variables like sum, which basically stops the sum function from working. I'm wondering where we should go with this.

Steps to Reproduce

1. See attached playground link

Expected Behavior

Uncertain at the moment, @nikku what do you think?