Implement simple custom columns for dynamic blocks

[libanp]

I've implemented a simple custom columns mechanism. There is a custom column type that takes a function and applies it to element. Though I notice there's a line saying "MAYBE: custom column functions", so I understand if this is something you'd rather not support or if you'd prefer a different approach.

It currently works like:

(defun my/org-ql:db-stats (element)
  (--when-let (org-element-property :raw-value element)
    (when (string-match "\\[\\([0-9]*\\(?:%\\|/[0-9]*\\)\\)\\]" it)
      (match-string 1 it))))

#+BEGIN: org-ql :query "todo: level:2" :columns (heading todo ((custom my/org-ql:db-stats) "Progress") ) 
#+END

Lambdas work too e.g by adding ((lambda (x) "hi") "example") to the :columns list