[BUG] Expression built-ins should be formalized

[Ph0enixKM]

Describe the bug
Right now lines built-in forces user to use it with parentheses:

let var = lines(file) // OK
let var = lines file // Error: Variable 'lines' does not exist

However this does not apply to len

let var = len(file) // OK
let var = len file // OK

What should be done?

• A more precise error message saying that expression built-ins should be used with parentheses
• Set the same constraints to other expression built-ins