Fix command substitutions run on the same line as a here-doc (#91)

When a command substitution is run on the same line as a here-document,
a syntax error occurs due to a regression introduced in ksh93u+ 2011-04-15:

true << EOF; true $(true)
EOF
syntax error at line 1: `<<EOF' here-document not contained within command substitution

The regression is caused by an error check that was added to make
the following script causes a syntax error (because the here-document
isn't completed inside of the command substitution):

$(true << EOF)
EOF

src/cmd/ksh93/sh/lex.c:
- Only throw an error when a here-document in a command substitution
  isn't completed inside of the command substitution.

src/cmd/ksh93/tests/heredoc.sh:
- Add a regression test for running a command substitution on the
  same line as a here-document.
- Add a missed regression test for using here-documents in command
  substitutions. This is the original bug that was fixed in ksh93u+
  2011-04-15 (it is why the error message was added), but a regression
  test for here-documents in command substitutions wasn't added in
  that version.

This bugfix was backported from ksh93v- 2013-10-10-alpha.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-07-23:
+
+- A command substitution that is run on the same line as a here-document
+  will no longer cause a syntax error.
+
 2020-07-22:
 
 - Fixed two race conditions when running external commands on