Add .editorconfig (#215)

Xcode 16 added support for EditorConfig files. This is a nice way to
manage per-project editor settings. Xcode 16 reads .editorconfig files
by default, but users can disable EditorConfig support in Xcode's
settings if they want.

This project's lint rules expect 2-space indentation. This change adds
an initial .editorconfig file that configures the indentation style plus
some whitespace settings.

Author: jparise

.editorconfig

@@ -0,0 +1,5 @@
+[*.swift]
+indent_style = space
+indent_size = 2
+insert_final_newline = false
+trim_trailing_whitespace = true