AK: Add GenericLexer::retreat()

This allows going back one character at a time, and then re-consume
previously consumed chars.
The code I need this for looks something like this:

    ASSERT(lexer.consume_specific('\\'));
    if (lexer.next_is("foo"))
        ...
    lexer.retreat();
    lexer.consume_escaped_character();  // This expects lexer.peek() == '\\'

AK/GenericLexer.h

@@ -48,6 +48,8 @@ public:
     bool next_is(StringView) const;
     bool next_is(const char*) const;
 
+    void retreat();
+
     char consume();
     bool consume_specific(char);
     bool consume_specific(StringView);