Add editorconfig file

The most important setting is to trim trailing whitespace, and use spaces not
tabs on the file system.

The rest can be argued. Indent size is taken from a (tiny) sammple of the
existing ghcide code base.

.editorconfig

@@ -0,0 +1,14 @@
+; This file is for unifying the coding style for different editors and IDEs.
+; More information at http://EditorConfig.org
+
+root = true
+
+[*]
+end_of_line = LF
+
+[*]
+indent_style = space
+indent_size = 4
+trim_trailing_whitespace = true
+insert_final_newline = true
+max_line_length = 80