Feat/137/v12 v13 support

[SimonMcKinsey]

@adamkleingit I've separated migration to small portions.
Angular v13 is now working with mobx newest version.
Fully tested.

However, I was forced to add to bank-v13 application the following:
makeAutoObservable(this);

Full snippet under account.store.ts:

  constructor() {
    if (localStorage.savedTransactions) {
      this.transactions = JSON.parse(localStorage.savedTransactions);
    }

    makeAutoObservable(this);

    autorun(() => {
      localStorage.savedTransactions = JSON.stringify(this.transactions);
    });
  }

Without it it wouldn't work.
I found the solution in this example: reactions.

Also, it seems like autorun is not encouraged with class component. Source: stackoverflow

I need a clarification on this one.
If everything seems legit, then I'll update the README as well with this new use case.

What am I missing here?

Thanks ahead.

Edit (06/11/2021): As I suspected, @EVASIVE suggested the same in his PR: #138

I'll update README

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