Validation for invalid assignment types in arguments

[volsa]

Describe the bug
We're missing a validation for invalid assignment statements in arguments (:=, =>), specifically the following should return an error

FUNCTION_BLOCK foo
    VAR_OUTPUT
        out_val : DINT;
    END_VAR
END_FUNCTION_BLOCK

FUNCTION main
    VAR
        foo_instance    : foo;
        out_val_local   : DINT;
    END_VAR

    foo_instance(out_val := out_val_local);
//                       ^^ should have been a `=>`
END_FUNCTION

With #1214 this will silently generate invalid code. For reference, CodeSys will return a "'out_val' is no input of foo" error here.

(I think the same should also happen for INPUT values, i.e. where a user typed => instead of :=, but CodeSys doesn't return an error for this)