Add .editorconfig (wads scaffolding standard)

Copied verbatim from a healthy repo in the ecosystem; the wads project
templates ship one and this repo predates that.

Claude-Session: https://claude.ai/code/session_01VipiLaG4xy7WctqY9w2475

Author: thorwhalen

.editorconfig

@@ -0,0 +1,17 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.{py,toml,yml,yaml}]
+indent_style = space
+indent_size = 4
+
+[*.md]
+trim_trailing_whitespace = false
+
+[Makefile]
+indent_style = tab