8134484: disallow backquotes as heredoc end marker delimiters

Reviewed-by: attila, sundar

nashorn/src/jdk.scripting.nashorn/share/classes/jdk/nashorn/internal/parser/Lexer.java

@@ -1453,7 +1453,7 @@ public class Lexer extends Scanner {
 
             // Scan identifier. It might be quoted, indicating that no string editing should take place.
             final char quoteChar = ch0;
-            final boolean noStringEditing = isStringDelimiter(quoteChar);
+            final boolean noStringEditing = quoteChar == '"' || quoteChar == '\'';
             if (noStringEditing) {
                 skip(1);
             }

nashorn/test/script/nosecurity/JDK-8073613.js

@@ -44,10 +44,6 @@ print(<<'EOM')
 ${a}${b}
 EOM
 
-print(<<`EOM`)
-${c}
-EOM
-
 print(<<"EOM")
 $\{a}
 EOM

nashorn/test/script/nosecurity/JDK-8073613.js.EXPECTED

@@ -1,5 +1,4 @@
 23
 ${a}${b}
 ${a}${b}
-${c}
 $\{a}