typeset -fp may print ill-formed command substitution

[magicant]

(Issue migrated from https://osdn.net/projects/yash/ticket/43410)

Describe the bug
typeset -fp prints command substitutions in the $() format even if the original function definition contained them in the backquoted form. In doing so, some quotations may not retain their exact semantics.

To Reproduce

$ foo() { `'`; }
$ typeset -fp foo
foo()
{
   $(')
}

$(') does not parse the same as the original backquoted command substitution.

Expected behavior
The printed function body should include the same backquoted command substitution.