Create .editorconfig

[Most major editors](https://editorconfig.org/#download) support EditorConfig for declaring per-file per-project indentation rules. It costs almost nothing to maintain, and it makes it easier to follow the coding styles already used in kitty.

.editorconfig

@@ -0,0 +1,14 @@
+root = true
+
+[*]
+indent_style = spaces
+indent_size = 4
+end_of_line = lf
+trim_trailing_whitespace = true
+
+[{Makefile,*.terminfo}]
+indent_style = tab
+
+# Autogenerated with tabs.
+[kitty/{unicode-data.c,emoji.wcwidth-std.h}]
+indent_style = tab