feat(beancount): add +beancount/occur command

Inspired by org-occur (or org-sparse-tree): hides all
transactions/directives in the buffer that don't involve a selected
account.

modules/lang/beancount/config.el

@@ -25,6 +25,7 @@
         :localleader
         "b" #'+beancount/balance
         "c" #'beancount-check
+        "s" #'+beancount/occur
         "l" #'beancount-linked
         "q" #'beancount-query
         "x" #'beancount-context