Move byobu config to XDG_CONFIG_HOME/byobu

Addresses #17
 - Moves `.tmux.conf` byobu config file to `.config/byobu` in this repo
 - Adjusts the symlink in Makefile

After this change, and if `BYOBU_CONFIG_DIR` uses `XDG_CONFIG_HOME`, it
is safe to re-link the config file and drop `~/.byobu` directory.

Makefile

@@ -46,7 +46,7 @@ RIPGREP_CONFIG_HOME=$(XDG_CONFIG_HOME)/rg
 endif
 
 ifndef BYOBU_CONFIG_DIR 
-BYOBU_CONFIG_DIR=~/.byobu
+BYOBU_CONFIG_DIR=$(XDG_CONFIG_HOME)/byobu
 endif
 
 ifndef SDKMAN_DIR
@@ -175,7 +175,7 @@ links: \
 		$(CFG_DIR)/.zlogout \
 		$(CFG_DIR)/.p10k.zsh
 	@{ \
-		for cfg in $$(find $(CFG_DIR)/.byobu $(CFG_DIR)/.config $(CFG_DIR)/.stack $(CFG_DIR)/.oh-my-zsh/custom -type f -not -name '*.dconf'); do \
+		for cfg in $$(find $(CFG_DIR)/.config $(CFG_DIR)/.stack $(CFG_DIR)/.oh-my-zsh/custom -type f -not -name '*.dconf'); do \
 			ln -svf $$cfg "$(HOME)$${cfg#$(CFG_DIR)}";\
 		done;\
 	}