Check if calling function is built-in or variable

[tyru]

#102 (comment)

echo func()

Currently vimlparser doesn't check if func is built-in function or local funcref variable.
Above script is passed with no errors because func may be a built-in function.

[tyru]

vimlparser provides VimLParser.builtin_functions.
now I think vimlparser should not check arity.

• If the member variable becomes old, vimlparser throws invalid syntax error
• vimlparser currently just parses syntax and doesn't do semantic analysis. now I no longer think it is really vimlparser's work 🤔