first step of go-gitea#12640

- add `side` and `line` to replace `new/old_position` in api like
  github
- add `start_line` in api like github for `multi-line comment` creating
- for multi-line comment, will cut all lines in range [start_line, line]
  for patch generate, then user can view all lines in this range on
  commnet list ui.
- not suport `start_side` now, `line` and `start_line` should in same
  side

todos:
- [ ] suport creating multi-line comments by ui & incomming mail
- [ ] more ui updates

Signed-off-by: a1012112796 <1012112796@qq.com>

…mment