Add .editorconfig

This helps with consistent indentation.
1 job for editorconfig in 2 minutes and 2 seconds
Name Stage Failure
build-debian-gcc Build
../src/chatty-popover-actions.c:23:1: error: ‘view_blist_cmd_show_in_contacts’ defined but not used [-Werror=unused-function]
view_blist_cmd_show_in_contacts (GSimpleAction *action,
../src/chatty-popover-actions.c:14:1: error: ‘view_msg_list_cmd_call’ defined but not used [-Werror=unused-function]
view_msg_list_cmd_call (GSimpleAction *action,
cc1: all warnings being treated as errors
ninja: build stopped: subcommand failed.
ERROR: Job failed: exit code 1