LibJS: Treat '\\' as an escaped character in template literals

Before this change we would ignore that the second backslash is escaped
and template strings ending with ` \\` would be unterminated as the
second slash was used to escape the closing quote.

Userland/Libraries/LibJS/Lexer.cpp

@@ -610,8 +610,15 @@ Token Lexer::next()
             consume();
             m_template_states.last().in_expr = true;
         } else {
+            // TemplateCharacter ::
+            //     $ [lookahead ≠ {]
+            //     \ TemplateEscapeSequence
+            //     \ NotEscapeSequence
+            //     LineContinuation
+            //     LineTerminatorSequence
+            //     SourceCharacter but not one of ` or \ or $ or LineTerminator
             while (!match('$', '{') && m_current_char != '`' && !is_eof()) {
-                if (match('\\', '$') || match('\\', '`'))
+                if (match('\\', '$') || match('\\', '`') || match('\\', '\\'))
                     consume();
                 consume();
             }

Userland/Libraries/LibJS/Tests/template-literals.js

@@ -7,6 +7,9 @@ test("plain literals with expression-like characters", () => {
 
 test("plain literals with escaped special characters", () => {
     expect(`foo\``).toBe("foo`");
+    expect(`foo\\`).toBe("foo\\");
+    expect(`foo\\\``).toBe("foo\\`");
+    expect(`foo\\\\`).toBe("foo\\\\");
     expect(`foo\$`).toBe("foo$");
     expect(`foo \${"bar"}`).toBe('foo ${"bar"}');
 });