Fix completion following $'foo\'bar'

On an interactive shell in emacs or vi, type a command with a $'…'
quoted string that contains a backslash-escaped single quote, like:

    $ true $'foo\'bar' ▁

Then begin to type the name of a file present in the current
working directory and press tab. Nothing happens as completion
fails to work.

The completion code does not recognise $'…' strings. Instead, it
parses them as '…' strings in which there are no backslash escapes,
so it considers the last ' to open a second quoted string which is
not terminated.

Plus, when replacing a $'…' string with a (backslash-escaped)
completed string, the initial '$' is not replaced:

    $ $'/etc/hosts<Tab>
    $ $/etc/hosts

src/cmd/ksh93/edit/completion.c:
- find_begin():
  - Learn how to recognise $'…' strings. A new local dollarquote
    flag variable is used to distinguish them from regular '…'
    strings. The difference is that backslash escapes (and only
    those) should be recognised as in "…".
  - Set a special type -1 for $'…' as the caller will need a way
    to distinguish those from '…'.
- ed_expand(): When replacing a quoted string, remove an extra
  initial character (being the $ in $') if the type set by
  find_begin() is -1.

Resolves: https://github.com/ksh93/ksh/issues/462

NEWS

@@ -9,6 +9,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   substitution after entering: '`something<TAB> or generate a spurious
   'end of file unexpected' syntax error after entering: '$(something<TAB>.
 
+- Fixed bug where tab completion stopped working for arguments following a
+  quoted string with an escaped backslash in the form $'foo\'bar'.
+
 2022-06-04:
 
 - Added a new --functrace long-form shell option which causes the -x/--xtrace