Generate PGN at position using tree-sitter allowing follow-minor-mode to work

[dekrueger]

These changes are intended to produce an appropriate PGN string at any position in the buffer.

Also, they append a * to each PGN, which seems to work around issues that occur when a result code is absent from the tree-sitter parse.

This appears to allow follow-minor-mode to function correctly.

But it does cause failure in the ert tests.

• pygn-mode-pgn-at-pos-01 fails because the actual game header is now returned at point-min.

• pygn-mode-pgn-at-pos-02 and pygn-mode-pgn-at-pos-05 fail because the PGN is no longer generated by truncating the buffer.

My apologies in advance for any elisp obtuseness. It's not something I've worked with often.

[rolandwalker]

Appreciated! Not obtuse! Would you like to comment out the failing tests in this PR? We can deal with them later.