Introduce more opinionated backspace/del/newline behavior

+ Instead of remapping delete-backward-char to doom/delete-backward-char
  (which was unreliable, depending on the mode), it is now overridden
  with it, without sacrificing its original functionality. The new
  behavior is as follows:
  + Fall back to sp-backward-delete-char when it makes sense to delete
    the adjacent pair: {|} => |
  + Collapse an indented pair block, if at bolp in between: {
      |
    } => {|}
  + Refresh a pair's :post-handlers when deleting into pair: {
      |
    } => {|} => {
      |
    } (can be repeated)
  + When cursor is preceded by whitespace, delete in increments of
    tab-width.
+ newline-and-indent has been advised to:
  + Only newline when in a string.
  + Continue comment lines consistently (needs more testing!)
  + Falls back to basic newline-and-indent, without affecting whitespace
    in the origin line (it would originally delete-horizontal-space
    before creating a new line).
+ Incorporates a set of reasonable defaults for brace expansion on RET
  or SPC, as mentioned in #343 and #413.

Affects #343, #413

modules/private/default/config.el

@@ -23,6 +23,51 @@
         epa-pinentry-mode 'loopback))
 
 
+;; disable :unless predicates with (sp-pair "'" nil :unless nil)
+;; disable :post-handlers with (sp-pair "{" nil :post-handlers nil)
+;; ...or specific :post-handlers with (sp-pair "{" nil :post-handlers '(:rem ("| " "SPC")))
+(after! smartparens
+  ;; Autopair quotes more conservatively; if I'm next to a word/before another
+  ;; quote, I likely don't want another pair.
+  (let ((unless-list '(sp-point-before-word-p
+                       sp-point-after-word-p
+                       sp-point-before-same-p)))
+    (sp-pair "'"  nil :unless unless-list)
+    (sp-pair "\"" nil :unless unless-list))
+
+  ;; Expand {|} => { | }
+  ;; Expand {|} => {
+  ;;   |
+  ;; }
+  (dolist (brace '("(" "{" "["))
+    (sp-pair brace nil
+             :post-handlers '(("||\n[i]" "RET") ("| " "SPC"))
+             ;; I likely don't want a new pair if adjacent to a word or opening brace
+             :unless '(sp-point-before-word-p sp-point-before-same-p)))
+
+  ;; Don't do square-bracket space-expansion where it doesn't make sense to
+  (sp-local-pair '(emacs-lisp-mode org-mode markdown-mode gfm-mode)
+                 "[" nil :post-handlers '(:rem ("| " "SPC")))
+
+  ;; Highjacks backspace to:
+  ;;  a) balance spaces inside brackets/parentheses ( | ) -> (|)
+  ;;  b) delete space-indented `tab-width' steps at a time
+  ;;  c) close empty multiline brace blocks in one step:
+  ;;     {
+  ;;     |
+  ;;     }
+  ;;     becomes {|}
+  ;;  d) refresh smartparens' :post-handlers, so SPC and RET expansions work
+  ;;     even after a backspace.
+  ;;  e) properly delete smartparen pairs when they are encountered, without the
+  ;;     need for strict mode.
+  ;;  f) do none of this when inside a string
+  (advice-add #'delete-backward-char :override #'doom/delete-backward-char)
+
+  ;; Makes `newline-and-indent' smarter when dealing with comments
+  (advice-add #'newline-and-indent :around #'doom*newline-and-indent))
+
+
 (when (featurep 'evil)
   (when (featurep! +evil-commands)
     (load! +evil-commands))