Existing logic already works well enough. It is enough to just add
these forms to the default values.

- let forms like if/when-let and letrec
- builtin define-inline also has the same syntax as defun
- try to highlight pcase-lambda and pcase-dolist like has already been
  done for pcase-let
- highlight variants of condition-case

While not all features of all forms* here are highlighted, this is
already more useful than not applying the highlighting to them.

*: like when-let and if-let's variable omitting