feat: Properties

[volsa]

This PR introduces the PROPERTY language feature. Properties in itself are getter and setter methods defined in a (stateful) POU and have the form of

FUNCTION_BLOCK fb
  VAR
    localPrivateVariable : DINT;
  END_VAR

  PROPERTY myProp : DINT
    GET
      // other statements potentially modifying the `localPrivateVariable`
      myProp := localPrivateVariable;
    END_GET
    
    SET
      // other statements potentially modifying internal variables
      localPrivateVariable := myProp;
    END_SET
  END_PROPERTY
END_FUNCTION_BLOCK

They are invoked by other POUs by simply accessing the property name. For example

FUNCTION main
  VAR
    x : DINT
    instance : fb;
  END_VAR

  instance.myProp := 5; // Internally resolves to `instance.__set_myprop(5)`
  x := instance.myProp; // Internally resolved to `x := instance.__get_myprop(5)`
END_FUNCTION

Specifically, references to properties always resolve to get calls with the exception in assignments when the property itself is accessed on the lhs.

The compiler will initially handle these properties as dedicated Property nodes in the CompilationUnit to have a 1:1 representation of the user code and only later on lower these properties to methods and property references to calls.

[mhasel]

There is an issue when accessing into an array-backing field directly. The getter logic is never called.

FUNCTION_BLOCK fb
    VAR
        localPrivateVariable : DINT := 5;
    END_VAR

    PROPERTY foo : ARRAY[0..9] OF STRING
        GET
            printf('getting$N');
        END_GET

        SET
            printf('setting$N');
        END_SET
    END_PROPERTY
END_FUNCTION_BLOCK


FUNCTION main
    VAR
        instance : fb;
    END_VAR
    instance.foo; // this prints 'getting'
    instance.foo[1] := 'hello world'; // this does not, it immediately accesses the array without calling the getter
    printf('%s$N', REF(instance.foo[1])); // same as above
END_FUNCTION

Another such example:

FUNCTION_BLOCK fb
    PROPERTY foo : ARRAY[0..9] OF DINT
        GET
            printf('getting foo$N');
        END_GET

        SET
            printf('setting foo$N');
        END_SET
    END_PROPERTY

    PROPERTY bar: INT
        GET
            printf('getting bar$N');
        END_GET

        SET
            printf('setting bar$N');
        END_SET
    END_PROPERTY
END_FUNCTION_BLOCK


FUNCTION main
    VAR
        instance : fb;
    END_VAR
    instance.foo[1] := 5;
    instance.bar := 1;
    printf('%d$N', instance.foo[instance.bar]); // the getter for `bar` is never called here
END_FUNCTION

[mhasel]

Mhh, this looks nice.