Shell: Recognise the (seemingly) bash-specific <<\WORD heredoc key

Bash eats the backslash in this format (similarly for W\ORD etc.).
Dr.POSIX doesn't specify this anywhere, but it's used all over the
place, so let's support it.

Userland/Shell/PosixLexer.cpp

@@ -168,7 +168,8 @@ Lexer::HeredocKeyResult Lexer::process_heredoc_key(Token const& token)
                 }
                 break;
             default:
-                if (escaped) {
+                // NOTE: bash eats the backslash outside quotes :shrug:
+                if (escaped && parse_state.last() != Free) {
                     builder.append('\\');
                     escaped = false;
                 }
@@ -568,6 +569,8 @@ ErrorOr<Lexer::ReductionResult> Lexer::reduce_start()
 
         auto contents = m_lexer.input().substring_view(start_index, end_index.value_or(m_lexer.tell()) - start_index);
         reconsume(contents);
+        if (end_index.has_value())
+            reconsume(m_lexer.input().substring_view_starting_after_substring(contents).substring_view(0, m_lexer.tell() - *end_index));
 
         m_state.buffer.clear();
         m_state.buffer.append(contents);