More flexible tree-sitter defun navigation

More flexible tree-sitter defun navigation

Before this change, treesit-beginning-of-defun skips nested defuns.
Now user can decide whether to skip nested defuns.

* lisp/treesit.el (treesit-search-forward-goto): Improve docstring.
(treesit-defun-prefer-top-level): New variable.
(treesit--defun-maybe-top-level): New function.
(treesit-beginning-of-defun)
(treesit-end-of-defun): Use treesit--defun-maybe-top-level.