Add higher level set macro to lisp

[vinc]

MOROS Lisp had var to bind a value to a symbol and set to rebind another value to the symbol (causing an error if the variable didn't exist).

This PR will rename set to mut because it is a low level operation to mutate a variable, and create a new higher level set macro that either bind a value to a symbol with var if the variable doesn't exist or rebind the new value with mut otherwise.

This behavior add consistency between MOROS Lisp and MOROS Shell.

• Rename set function to mut
• Add var? function
• Add set macro
• Update doc
• Update example scripts

[vinc]

I'm working on adding def to MOROS Shell (#752) and with this PR they will both have the same def and set operations.