Allow double quotes within backtick comsub within double quotes

The following reproducer causes a spurious syntax error:

    foo="`: "("`"

The nested double quotes are not recognised correctly, causing a
syntax error at the '('. Removing the outer double quotes (which
are unnecessary) is a workaround, but it's still a bug as every
other shell accepts this. This bug has been present since the
original Bourne shell.

src/cmd/ksh93/sh/lex.c: sh_lex(): case S_QUOTE:
- If the current character is '"' and we're in a `...` command
  substitution (ingrave is true), then do not switch to the old
  mode but keep using the ST_QUOTE state table.

Thanks to @JohnoKing for the report and to @atheik for the fix.

Co-authored by: Martijn Dekker <martijn@inlv.org>
Resolves: https://github.com/ksh93/ksh/issues/352

src/cmd/ksh93/tests/quoting2.sh

@@ -290,5 +290,10 @@ got="${var:+'}text between expansions${var:+'}"
 [[ $got == "$exp" ]] || err_exit "Single quotes misparsed in expansion operator string (8)" \
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 
+# ======
+# https://github.com/ksh93/ksh/issues/352
+got=$(eval 'foo="`: "^Exec(\[[^]=]*])?="`"' 2>&1) || err_exit "Backtick command substitutions can't nest double quotes" \
+	"(got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))