feat(emacs/lean-mode): bind "C-c C-k" to quail-show-key

 - Use this to see how to type a unicode character
fix(emacs/lean-syntax): change comment symbols to be considered as punctuations

Please read the documentation of "modify-syntax-entry":

"The first character of NEWENTRY should be one of the following:
  _           symbol constituent
  .           punctuation
"

Close: #306