Add explicit config for EditorConfig and Prettier

For some reason in my editor, trailing commas keep getting added. Add
explicit configuration for Prettier to ensure this doesn't happen, and
go ahead and add EditorConfig while we're at it for the same reasons
(and because Prettier reads EditorConfig's config).

.editorconfig

@@ -1,2 +1,15 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+
+[*.{rb,js,json,yaml,yml}]
+indent_style = space
+indent_size = 2
+
+[*.{rb,js}]
+max_line_length = 80
+
 [README.md]
 max_line_length = unset

.prettierrc.json

@@ -0,0 +1,5 @@
+{
+  "bracketSpacing": true,
+  "trailingComma": "none",
+  "semi": true
+}