Tree-sitter indentation for js-mode & cc-mode

[Yuan Fu]

I did some work to allow tree-sitter indentation engine to plug in to 
c-offset-alist. Currently in a tree-sitter indent rule, we have

(MATCHER ANCHOR OFFSET)

OFFSET is normally an integer, but now it can also be a syntax symbol 
recognized by cc-mode’s indentation engine. In that case, tree-sitter indent 
calculates the indent using c-calc-offset, passing the syntax symbol and anchor 
position to it, and c-calc-offset will give us the integer offset based on 
c-offset-alist.

I’ve written indent rules for js-mode, they are in js-treesit-cc-indent-rules. 
Overall it works pretty well. Theo, could you give it a try? From my testing it 
is already an improvement from the original rules. I didn’t finish the JSX part 
and just copied your original rules for JSX. In the future I can probably port 
that to cc-style too.

I also added imenu support for js-mode and ts-mode, and navigation for 
python-mode.

Yuan