Handle special elements of imenu--index-alist that provides function to call

[muffinmad]

From imenu--index-alist docstring:

Special elements look like
(INDEX-NAME POSITION FUNCTION ARGUMENTS...).
To "go to" a special element means applying FUNCTION to
INDEX-NAME, POSITION, and the ARGUMENTS.

Eglot creates this kind of imenu--index-alist:

(("Function"
  (#1="foo"
      [(:containerName nil :kind 12 :name #1# :location
		       (:range
			(:start
			 (:line 0 :character 0)
			 :end
			 (:line 2 :character 0))
			:uri "file:///Users/mad/workspace/test/test.py"))]
      #f(compiled-function
	 (name one-obj-array)
	 #<bytecode -0x17f965a3c21ddd45>))))

Calling imenu-list-smart-toggle lead to this error:

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p [(:containerName nil :kind 12 :name "foo" :location (:range (:start (:line 0 :character 0) :end (:line 2 :character 0)) :uri "file:///Users/mad/workspace/test/test.py"))])
  imenu-list-<=(#<marker at 1 in test.py> [(:containerName nil :kind 12 :name "foo" :location (:range (:start (:line 0 :character 0) :end (:line 2 :character 0)) :uri "file:///Users/mad/workspace/test/test.py"))] #<marker at 1 in test.py>)
  imenu-list--current-entry()
  imenu-list--show-current-entry()
  imenu-list-update(nil t)
  imenu-list-minor-mode(1)
  imenu-list-smart-toggle()
  funcall-interactively(imenu-list-smart-toggle)
  call-interactively(imenu-list-smart-toggle record nil)
  command-execute(imenu-list-smart-toggle record)
  execute-extended-command(nil "imenu-list-smart-toggle" nil)
  funcall-interactively(execute-extended-command nil "imenu-list-smart-toggle" nil)
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)

[bmag]

Continued in #63.