feat: support 'this' type on methods and properties

[mattjohnsonpint]

Fixes #2904

Changes proposed in this pull request:

• Fixes support for polymorphic this type on class methods and properties
• Reports a "not implemented" for fields declared with the this type (would be nice, but is complicated)
• Adds compiler tests for this feature

To clarify, the this type was already implemented, but was returning the base type instead of the calling instance type.

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[mattjohnsonpint]

FYI, I considered trying to bind the prototype instance to the derived type, but I couldn't quite get that way to work. So instead, I resolve the current "this" to use as the class instance instead. It works, but let me know if there's a better way to approach this. Thanks.

[HerrCai0907]

Could we create an override method for inheritance class
e.g.
for class A extends B, B has method foo return this, we resolve it as return B. A will generate override method foo return A.
Then resolve function should resolve to correct function.

[mattjohnsonpint]

Could we create an override method for inheritance class ...

I explored this a bit. It would have side effects, even without using the this return type.

For example, given:

class X {
  private foo: i32 = 0;

  doSomething(): void {
    this.foo = 1;
  }
}

class Y extends X {
}

... we can't just transform it to this:

class X {
  private foo: i32 = 0;

  doSomething(): void {
    this.foo = 1;
  }
}

class Y extends X {
  doSomething(): void {
    this.foo = 1;
  }
}

... because field foo is declared private.

The resolver implementation does effectively create an alias of the member, but in doing so it uses the original prototype, bound to the base instance. It doesn't create an actual override.