Validate "wrapped" assignment statements OR treat assignments as expressions

[mhasel]

The following example of statements currently fails during codegen:

PROGRAM exp
VAR
    x, y : INT;
END_VAR
    +x := 1 + 4;                 // codegen err  | `UnaryExpression { op+,  Assignment {...}`
     x + y := 1;                 // codegen err  | `BinaryExpression { op+, Reference,,  Assignment {...} }`
END_PROGRAM

Trying to compile this will fail during the codegen stage, with e.g. :

no type hint available for Assignment {
    left: ReferenceExpr {
        kind: Member(
            Identifier {
                name: "y",
            },
        ),
        base: None,
    },
    right: LiteralInteger {
        value: 1,
    },
} 

We should either properly validate against these statements or start treating assignments as expressions (rel.: https://github.com/PLC-lang/rusty/issues/1173).