build: add editorconfig and prettierrc

Problem: no editor or formatter configuration, inconsistent with
cp.nvim and diffs.nvim conventions.

Solution: add .editorconfig (2-space Lua indent, utf-8, final newline)
and .prettierrc (prose wrap, 80 cols, single quotes, no semi) matching
the other repos.

.editorconfig

@@ -0,0 +1,9 @@
+root = true
+
+[*]
+insert_final_newline = true
+charset = utf-8
+
+[*.lua]
+indent_style = space
+indent_size = 2

.prettierrc

@@ -0,0 +1,9 @@
+{
+  "proseWrap": "always",
+  "printWidth": 80,
+  "tabWidth": 2,
+  "useTabs": false,
+  "trailingComma": "none",
+  "semi": false,
+  "singleQuote": true
+}