Define sexp-default thing to improve navigation in Emacs 31

[rrudakov]

Details here: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=78458

This PR fixes a few issues, mentioned in #97, to fix all navigation issues, the best way would be to switch to a modified grammar (see sogaiu/tree-sitter-clojure#65).

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• The commits are consistent with our contribution guidelines.
• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
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Issues with navigation are already mentioned in the README, including improvements in Emacs 31.

Thanks!