Handle escaped expansions in heredocs

Fixes #28

corpus/commands.txt

@@ -169,3 +169,15 @@ wc -l $tmpfile
     (command_name (word))
     (word)
     (simple_expansion (variable_name))))
+
+======================================
+Heredocs with escaped expansions
+======================================
+
+cat  << EOF
+DEV_NAME=\$(lsblk)
+EOF
+
+---
+
+(program (redirected_statement (command (command_name (word))) (heredoc_redirect (heredoc_start))) (heredoc_body))

src/scanner.cc

@@ -61,6 +61,13 @@ struct Scanner {
           return did_advance;
         }
 
+        case '\\': {
+          did_advance = true;
+          advance(lexer);
+          advance(lexer);
+          break;
+        }
+
         case '$': {
           lexer->result_symbol = middle_type;
           return did_advance;