unindent-string.el

(require 's)

(defun count-leading-whitespace (str)
  "Count leading indentation chars in STR."
  (let ((trimmed (string-trim-left str)))
    (- (length str) (length trimmed))))

(defun unindent-string (str)
  "Remove common leading whitespace from each line in STR."
  (let* ((str-as-lines (s-lines str))
         (n-to-trim (apply #'min
                           (mapcar #'count-leading-whitespace str-as-lines))))
    (mapconcat 'identity
               (mapcar (lambda (s) (substring s n-to-trim)) str-as-lines)
               "\n")))