Spell checker property to allow checking uppercase

# Rationale
Existing [SpellCheck class](https://docs.microsoft.com/en-us/dotnet/api/system.windows.controls.spellcheck?view=netcore-3.1) used in `System.Windows.Controls.TextBox` and `System.Windows.Controls.RichTextBox` does not check words written uppercase by design. However, there are many use cases such as business requirements, message formats by industry standards/conventions et cetera, which require uppercase texts to be checked. A property to change this behavior should be available to the developers.

# Suggested Change
In addition to current [API](https://github.com/dotnet/wpf/blob/ae1790531c3b993b56eba8b1f0dd395a3ed7de75/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/SpellCheck.cs), a property called `IgnoreUppercase` with `true` default value would allow developers control behavior of WPF Controls.