[swift2objc] Support Computed Properties and Property Observers (i.e `get`, `set`, `willSet` and `didSet`)

[nikeokoronkwo]

This issue is to implement support for Computed Properties and Property Observers

Computed Properties contain syntax for getting and setting properties in a compound declaration: get and set.

When it comes to Property Observers, they are called when properties are to be assigned. There are two: willSet and didSet.
From https://docs.swift.org/swift-book/documentation/the-swift-programming-language/properties

willSet is called just before the value is stored.
didSet is called immediately after the new value is stored.

// getters and setters in Protocols
protocol Account {
    var balance: Double { get set } // Get and Set
    var accountType: String { get } // Get only

    func displayDetails()
}

// getters and setters in Structs
class CheckingAccount: Account {
    var balance: Double {
        get {
            return internalBalance
        }
        set {
            internalBalance = newValue
            print("Balance updated to \(internalBalance)")
        }
    }

    func displayDetails() {
        print("Account Type: \(accountType), Balance: \(balance)")
    }
}

Getters and setters in structures and classes are usually already defined before-hand, while those in Protocols are not.