Add paredit function `paredit-backward-kill-word`

As an Emacs user, I am accustomed to using the "paredit-backwards-kill-word" function to delete words backwards while preserving parentheses. This function doesn't appear to be available in Calva (unless I'm missing something?).

My VSCode-fu is not up to the task of porting the Emacs code to VSCode but I've included the Emacs implementation below for reference.

Many thanks!

http://paredit.org/releases/26/paredit.el

```elisp
(defun paredit-backward-kill-word ()
"Kill a word backward, skipping over any intervening delimiters."
(interactive)
(if (not (or (bobp)
(eq (char-syntax (char-before)) ?w)))
(let ((end (point)))
(backward-word 1)
(forward-word 1)
(goto-char (min end (point)))
(let* ((parse-state (paredit-current-parse-state))
(state
(paredit-kill-word-state parse-state 'char-before)))
(while (and (< (point) end)
(progn
(setq parse-state
(parse-partial-sexp (point) (1+ (point))
nil nil parse-state))
(or (eq state
(paredit-kill-word-state parse-state
'char-before))
(progn (backward-char 1) nil)))))
(if (and (eq state 'comment)
(eq ?\# (char-after (point)))
(eq ?\| (char-before (point))))
(backward-char 1)))))
(backward-kill-word 1))
```

Use the linked paredit.el implementation as the behavioral reference, then locate Calva's existing paredit commands and their tests or command registrations. Confirm how backward word deletion currently works, implement the missing command while preserving parentheses, and verify deletion across delimiters and ordinary words.

Written by the indexing model from the issue text.