Introduce an .editorconfig.

Follow up on 5531228, which removed
Prettier, with some simple rules in a format that's understood by most
editors and some opinionated code formatters (like Prettier).

This should help keep less obvious things - like tabs vs spaces and CRLF vs
LF - from working their way into the codebase, but stops short of making
more destructive formatting changes.

.editorconfig

@@ -0,0 +1,10 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+max_line_length = 80