fix: blame incompatible neovim function

[IIIHSUAN]

This PR addresses #1405 and #1388, which are caused by the incompatible function signature vim.str_utfindex in Neovim v0.11.

• vim.str_utfindex before 0.11:

  --- @param str string
  --- @param index? integer
  --- @return integer UTF-32 index
  --- @return integer UTF-16 index
  function vim.str_utfindex(str, index) end

• vim.str_utfindex since 0.11:

  --- Parameters:
  --- {s} (string)
  --- {encoding} ("utf-8"|"utf-16"|"utf-32")
  --- {index} (integer)
  --- {strict_indexing} (boolean?) default: true
  --- Return:
  --- (integer)
  vim.str_byteindex(s, encoding, index, strict_indexing)

At first, I considered simply using a helper function to wrap the incompatible functions, but that felt too tedious for such a common task. Then I found the vimscript API function vim.fn.strdisplaywidth, which seems to do the job and makes more sense in this context.

You can find these implementations in the two separate commits. Both commits work fine.

Vim functions: strchars vs strwidth vs strlen vs strdisplaywidth for reference.

Also, many thanks to Lewis. Gitsigns is awesome!