Use which-function to find the current function name

[dgchurchill]

Hi, thanks for this package! Here's a small update to find the current method/function name using which-function. This should work for any Treesitter mode (e.g. csharp-ts-mode) because they hook up treesit-add-log-current-defun to find the function name. It should work elsewhere too and I think it's the standard way of finding the function name.

[sebasmonia]

Thanks for your PR! And glad Sharper is useful for you.

I will test the code and report back - my sense is that non-TS users will still need an alternative. But maybe (which-function) really is enough on its own!