>>     export const add = (a, b) -!- => a + b;
>>
>> typing 'C-M-f' should jump to the end of the next sexp
>> (to the end of whole "binary_expression"):
>>
>>     export const add = (a, b) => a + b-!-;
>>
>> since only tree-sitter knows about "binary_expression",
>> so 'forward-sexp-default-function' can't be used here.
>
> Actually, I have one idea of possible heuristics:
>
> 1. first try 'forward-sexp-default-function'
> 2. if it crosses the boundary of sexp defined by 'treesit-thing-settings'
>    then use 'treesit-end-of-thing' instead
>
> This should work.  Ok, will try.

This is implemented now in the attached patch, and it works nicely.

The main rule is the following: 'forward-sexp-default-function'
should not go out of the current thing, neither go inside a sibling.
So we use 'treesit-end-of-thing' in such cases.  But when inside
a thing or outside a thing, use the default function.

This supposes that such things as "identifier" in js should be
removed from 'treesit-thing-settings' since identifiers should be
navigated the same way as such keywords as "export" and "const"
using 'forward-sexp-default-function'.

What should remain in 'treesit-thing-settings' are only grouping
constructs such as "parenthesized_expression" and "statement_block".

Removing "identifier" from 'treesit-thing-settings' exposed a problem
in 'treesit-navigate-thing'.  This line

  ((and (null next) (null prev)) parent)

tries to go out of the current thing to its parent,
thus breaking the main principle that 'forward-sexp'
should move forward across siblings only.  But removing
this line fixed the problem: