Compiler/type checker: Support for inheritance needed

https://share.sourceacademy.org/yezxq
currently produces:
```
SyntaxError: SyntaxError: Expected " ", "/*", "//", "\r\n", "{", [ \t\x0C], or [\r\n] but "e" found.
```
The program:
```
class Parent {
    void test(Parent p) {
    }
}

class Test extends Parent {
    public static void main(String[] args) {
        
    }
}
```