Add editorconfig

[editorconfig][0] is a cross-editor config file that helps define coding
styles. I find this useful in almost all my projects with mixed
languages b/c convincing my editor to conform to each project's specific
style is a huge pain.

[0]: https://editorconfig.org/

Signed-off-by: Daniel Xu <[email protected]>

Author: danobi

.editorconfig

@@ -0,0 +1,5 @@
+root = true
+
+[*.[ch]]
+indent_size = 8
+indent_style = tab