Add missing license text for markdown_parser.leg

[musicinmybrain]

The file examples/markdown/grammar/markdown_parser.leg is copied from https://github.com/jgm/peg-markdown/blob/0.4.14/markdown_parser.leg, and it contains the following notice.

TatSu/examples/markdown/grammar/markdown_parser.leg

Lines 4 to 14 in 68ed4a8

	markdown_parser.leg - markdown parser in C using a PEG grammar.
	(c) 2008 John MacFarlane (jgm at berkeley dot edu).
	This program is free software; you can redistribute it and/or modify
	it under the terms of the GNU General Public License or the MIT
	license. See LICENSE for details.
	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	GNU General Public License for more details.

However the actual copyright and license notice for the MIT option are missing, even though the MIT license requires them to be redistributed. Similarly, the license notice for the GPL-2.0-or-later option is missing.

To remedy these problems, this PR includes a copy of https://github.com/jgm/peg-markdown/raw/refs/tags/0.4.14/LICENSE. The parts about peg-0.1.4 and my_getopt are not relevant, as the corresponding files from https://github.com/jgm/peg-markdown are not included here.