Replace :xref-backend setting with :jump

Which takes :definitions, :references, :documentation and :xref-backend.
Each accepts a function that will jump to definitions, references, and
documentation respectively.

If a major-mode has an :xref-backend, you don't need :definitions or
:references.

modules/private/hlissner/+bindings.el

@@ -292,6 +292,7 @@
  :m  "gT" #'+workspace/switch-left
  :m  "gd" #'+jump/definition
  :m  "gD" #'+jump/references
+ :m  "gh" #'+jump/documentation
  :n  "gp" #'+evil/reselect-paste
  :n  "gr" #'+eval:region
  :n  "gR" #'+eval/buffer