[Proposal] Organize partial classes inside Controls (#13113)

### Description of Change

- Move all related files into the same folder
- Move all the code inside ".impl.cs" into "<file>.Controls.cs"
- It doesn't seem like it's been useful to have these concepts
separated. We also already have some repeats with the name "<file>.cs"
because of platform specifics so I'm thinking that having the suffix
"controls.cs" which is related to all things bindable is useful
- Rename the file with the mappers to "Entry.Mapper.cs". It's annoying
having 3 things called `Entry.cs` now we'll just have 2 :-)

<img width="160" alt="image"
src="https://user-images.githubusercontent.com/5375137/216784012-f1ba6538-5cca-4e4e-9a79-6143540a27cd.png">


### Issues Fixed

Headaches and confusion

src/Controls/src/Core/Entry.cs → src/Controls/src/Core/Entry/Entry.Controls.cs

@@ -9,7 +9,7 @@ namespace Microsoft.Maui.Controls
 	/// <summary>
 	/// Entry is a single line text entry. It is best used for collecting small discrete pieces of information, like usernames and passwords.
 	/// </summary>
-	public partial class Entry : InputView, IFontElement, ITextAlignmentElement, IEntryController, IElementConfiguration<Entry>
+	public partial class Entry : InputView, IFontElement, ITextAlignmentElement, IEntryController, IElementConfiguration<Entry>, IEntry
 	{
 		/// <summary>
 		/// Backing store for the <see cref="ReturnType"/> property.
@@ -299,5 +299,12 @@ public IPlatformElementConfiguration<T, Entry> On<T>() where T : IConfigPlatform
 		void ITextAlignmentElement.OnHorizontalTextAlignmentPropertyChanged(TextAlignment oldValue, TextAlignment newValue)
 		{
 		}
+
+		Font ITextStyle.Font => this.ToFont();
+
+		void IEntry.Completed()
+		{
+			(this as IEntryController).SendCompleted();
+		}
 	}
 }