File scoped namespaces (#3529)

[Language Reference](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-10.0/file-scoped-namespaces)

Updates all the files, one less level of indentation.

Some small changes were made to API surfaces, renaming `PKM pkm` -> `PKM pk`, and `LegalityAnalysis.pkm` -> `LegalityAnalysis.Entity`

PKHeX.WinForms/Controls/Slots/DropModifier.cs

@@ -1,9 +1,8 @@
-namespace PKHeX.WinForms.Controls
+namespace PKHeX.WinForms.Controls;
+
+public enum DropModifier
 {
-    public enum DropModifier
-    {
-        None,
-        Overwrite,
-        Clone,
-    }
-}
+    None,
+    Overwrite,
+    Clone,
+}