Enable `strictPropertyInitialization`

[alexdima]

We have enabled strictNullChecks, but the TS compiler doesn't catch the following:

class A {
  prop: number;
  constructor() {
    // forgot to assign to prop, so it will be undefined...
  }
}

Possible fixes:

class A {
  prop: number;
  constructor() {
    this.prop = 5; // Assign the property in the ctor
  }
}

class A {
  prop!: number; // use exclamation mark to suppress the error
  constructor() {
    this._init();
  }
  private _init() {
    this.prop = 5;
  }
}

---

In order to get errors for such cases, we need to add "strictPropertyInitialization": true to our tsconfig.json.

@microsoft/vscode it would be nice if we could tackle some of these errors during debt week.

[edit 11.11]: There are 8 errors left:

• vs/base/browser/ui/tree/asyncDataTree.ts @joaomoreno
• src/vs/workbench/api/common/extHostTypes.ts @jrieken
• src/vs/workbench/api/common/extHostWebview.ts @mjbvz

[isidorn]

Adding it to next milestone so it shows up in my issue query for debt week.
If we do not tackle it completely we can always move it to the next one afterwards.