[Proposal] Customized Auto Properties

I introduced many suggestions to shorten properties syntax, as they are the largest VB blocks. Auto properties are grate, but there are many cases where they can't be used. Here is one of them:
I had to write the full property body, just to call a sub in the Set section:
```VB.NET
Dim _account As Account
Public Property Account As Account
Get
Return _account
End Get
Set(value As Account)
_account = value
Refresh()
End Set
End Property
```

I suggest to allow to append some code to auto implemented properties, as this:
```VB.NET
Public Property Account As Account
On Set
Refresh()
End Set
```

which will be lowered to the first example. and in case of one statement set, it can be reduced to:
```VB.NET
Public Property Account As Account On Set: Refresh()
```

The same can be done with `On Get` if needed. There is no need to use both together, as in this it will be logical to use the normal full property body, and I had [a previous proposal](#567) to make it shorter for
a single-line Get or Set.