editor/reformat: refactor & fixes

+ Change +format-type to +format-region-p (now a boolean)
+ Add PRESERVE-INDENT-P boolean argument to +format-buffer
+ Add +format-preserve-indentation variable (only controls indent
  preservation during +format/buffer, not +format/region, where it's
  always enabled).
+ Fix error arising from +format|buffer hook when no formatter is
  defined for the current mode #893
+ Change +format|buffer to alias for +format/buffer.

modules/lang/go/config.el

@@ -15,7 +15,7 @@
   ;; Redefines default formatter to *not* use goimports if reformatting a
   ;; region; as it doesn't play well with partial code.
   (set-formatter! 'gofmt
-    '(("%s" (if (or (eq +format-type 'region)
+    '(("%s" (if (or +format-region-p
                     (not (executable-find "goimports")))
                 "gofmt"
               "goimports"))))