Feat: Improve focus/first responder handling in SwiftUI

[marinofelipe]

Why?

To provide a better API for making CurrencyTextField the first responder in SwiftUI.

Context

CurrencyTextField is currently bridged from UIKit to SwiftUI, so iOS 15's focus API doesn't work for it.
The only way to make a CurrencyTextField the first responder programmatically is as indicated by YeungKC on #81:

textFieldConfiguration: { uiTextField in
    DispatchQueue.main.async {
        if !becomeFirstResponderCalled {
            uiTextField.becomeFirstResponder()
        }
        becomeFirstResponderCalled = true
    }
}

which is error prone and ugly.

Changes

This PR adds a new property to CurrencyTextFieldConfiguration called hasFocus, an OptionalBinding to drive an listen to the first responder state of a CurrencyTextField wrapped UITextField.

The usage is as follows:

struct SwiftUIExampleView2: View {
    @State private var hasFocus: Bool? = false // or this initially as true
    @State private var text = ""

    var body: some View {
        CurrencyTextField(
            configuration: .init(
                placeholder: "type here...",
                text: $text,
                hasFocus: $hasFocus,
                formatter: .default,
                textFieldConfiguration: { uiTextField in
                    uiTextField.borderStyle = .roundedRect
                    uiTextField.font = UIFont.preferredFont(forTextStyle: .body)
                },
                onEditingChanged: { isEditing in
                    print("onEditingChanged: \(isEditing)")
                },
                onCommit: {
                    print("onCommit")
                }
            )
        )
        .onAppear {
            hasFocus = true
        }
    }
}

Tests

Tests were updated and CurrencyTextFieldConfigurationTests was extended.

Related issue: #81

[codecov-commenter]

Codecov Report

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