Update README.md, Update missing docs, Fix pub.dev Health suggestions and Analysis suggestions, v3.4.0+1

Fixed HttpAuthCredentialDatabase.removeHttpAuthCredential on Android, Fixed some cases where takeScreenshot was not working on Android, fix #390, Updated HttpAuthCredentialDatabase.getAllAuthCredentials method return type

added getScrollX and getScrollY webview methods, added HttpOnly and SameSite set cookie options, added animated option to scrollTo and scrollBy webview methods, Added error and message to the ServerTrustChallenge class for iOS, added contentInsetAdjustmentBehavior webview iOS-specific option, added getCertificate android-specific webview method, added copy and copyWithValue methods for webview class options

added new webview methods, added supporZoom webview option on iOS, bug fixes, prepare new version 3.4.0

added clearHistory webview methods on Android, setContextMenu and clearFocus webview methods, added ContextMenuOptions class

Added 'toString()' method to various classes in order to have a better output instead of simply 'Instance of ...', updated getOptions() method

added onPageCommitVisible event, updated dart code for new events and methods, updated docs, fix #367

Updated Android context menu workaround, updated iOS onCreateContextMenu event, Added Android keyboard workaround to hide the keyboard when clicking other HTML elements

expiresDate option (#338)

added HeadlessInAppWebView class, added close, addMenuItem and addMenuItems methods to ChromeSafariBrowser, added ChromeSafariBrowserMenuItem class, fixed InAppWebView.channel null exception on android, fix #305, fix #245, fix #299

fix scrollbar on iOS always show if not disable scroll

Fix crash when `prompt` was called on Android Q.

Fix crash on nil/invalid URL (iOS)