feat(otp): XDG basedir support, or user value for OTP_HOME

otp plugin now uses the following logic for its home:
- If $OTP_HOME is set by the user, use that value
- If ~/.otp dir already exists, use that value
- If XDG_DATA_HOME is set, use $XDG_DATA_HOME/otp
- Else fallback to ~/.otp

plugins/otp/otp.plugin.zsh

@@ -1,4 +1,10 @@
-export OTP_HOME=~/.otp
+if [[ -z "$OTP_HOME" ]]; then
+  if [[ ! -d "$HOME/.otp" ]] && [[ -n "$XDG_DATA_HOME" ]]; then
+    export OTP_HOME="$XDG_DATA_HOME/otp"
+  else
+    export OTP_HOME=~/.otp
+  fi
+fi
 mkdir -p $OTP_HOME
 
 function ot () {