chore: add .editorconfig and .nvmrc for consistent DX

.editorconfig

@@ -0,0 +1,21 @@
+# EditorConfig — consistent formatting across editors
+# https://editorconfig.org
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = space
+indent_size = 2
+
+[*.md]
+trim_trailing_whitespace = false
+
+[*.{yml,yaml}]
+indent_size = 2
+
+[Makefile]
+indent_style = tab