Add .editorconfig for csharp directory

Nicer experience for e.g. VS Code

csharp/.editorconfig

@@ -0,0 +1,18 @@
+# Remove the line below if you want to inherit .editorconfig settings from higher directories
+root = true
+
+# C# files
+[*.cs]
+
+#### Core EditorConfig Options ####
+
+# Indentation and spacing
+indent_size = 4
+indent_style = space
+tab_width = 4
+
+# New line preferences
+# See https://github.com/protocolbuffers/protobuf/issues/9526
+#end_of_line = lf
+insert_final_newline = false
+trim_trailing_whitespace = true