style(editorconfig): add EditorConfig file for consistent style

Add an EditorConfig file to ensure all editors know how to edit files
correctly. Apply conservative settings based on the existing files. Rust
options are based on the defaults of `rustfmt`.

.editorconfig

@@ -0,0 +1,13 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+charset = utf-8
+insert_final_newline = false
+
+[*.rs]
+max_line_length = 100
+
+[*.md]
+max_line_length = 100