Add improved tree-sitter navigation

This new set of functions (and tests) should eliminate
defun-navigation bugs and limitations we currently have.  This commit
doesn't change any existing bahavior: treesit-beginning/end-of-defun
and friends are unchanged.  The plan is to later switch gear and
replace the current functions with the new ones introduced in this
change.

This is a relatively big change, but I've setup a comprehensive test,
and it should fix current bugs, so I think it's ok to put it on the
release branch.

The gist of the new navigation is to use treesit--defuns-around to
find the previous sibling defun, next sibling defun, and the parent
defun, then use this information to move to previous/next
beginning/end of defun in treesit--navigate-defun.

I also added comprehensive testing that tests all four possible
operations (prev-beg, next-beg, prev-end, next-end) starting at all
possible positions (between two sibling defuns, inside a sibling
defun, etc).

* lisp/treesit.el (treesit-defun-type-regexp): Expand definition to
allow (REGEXP . FILTER).  Old functions don't support this, but it
should be fine since we are soon replacing them.

(treesit-defun-tactic)
(treesit-defun-skipper): New variables.

(treesit-default-defun-skipper)
(treesit--defuns-around)
(treesit--top-level-defun)
(treesit--navigate-defun): New functions.

* test/src/treesit-tests.el (treesit--ert-insert-and-parse-marker)
(treesit--ert-collect-positions)
(treesit--ert-test-defun-navigation): New helper functions.

(treesit--ert-defun-navigation-python-program)
(treesit--ert-defun-navigation-js-program)
(treesit--ert-defun-navigation-bash-program)
(treesit--ert-defun-navigation-nested-master): New variables.

(treesit-defun-navigation-nested-1)
(treesit-defun-navigation-nested-2)
(treesit-defun-navigation-nested-3)
(treesit-defun-navigation-top-level): New tests.

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