Minor improvement for tree-sitter explorer

Minor improvement for tree-sitter explorer

If you open an empty python buffer and type

1 + 2
a
b

Currently the explorer only displays the top-level node at point, ie,
only 1 + 2, only a, or only b. That's kind of awkward, so if the
buffer is small, show the entire parse tree.

* lisp/treesit.el (treesit--explorer-refresh): See above.