Hi Yuan!

I was thinking that a quick way to enable indentation styles and
customizable indentation styles is to also accept a function in
tree-sitter-simple-indent.

What do you think of the suggested patch?

That would allow a defcustom such as
```
(defcustom c-ts-mode-indent-rule-function #'c-ts-mode--some-specific-style
  "Indentation style of choice"
  :group 'c)
```

and in the major-mode init:
```
    (setq-local treesit-simple-indent-rules c-ts-mode-indent-rule-function)
```

-- 
Theo