Add .editorconfig (emscripten-core#8359)

Mainly so that editors know that this project uses 2 space indents
for Python instead of the more typical 4.

Author: msabramo

.editorconfig

@@ -0,0 +1,14 @@
+# https://EditorConfig.org
+
+# top-most EditorConfig file
+root = true
+
+# Unix-style newlines with a newline ending every file
+[*]
+end_of_line = lf
+insert_final_newline = true
+
+[*.{js,py}]
+charset = utf-8
+indent_style = space
+indent_size = 2