fix: converting tabs to spaces in virtual text

[luozhiya]

• getcurpos

  • Returns [0, lnum, col, off, curswant]

  • The curswant number is the preferred column when moving the cursor vertically.

  • We can simplify the logic because there is no vertical manipulation involved in code completion.
  • Also when feeding tab, getcurpos()[2] + 1 is not the correct number of columns.
• prop_add

  • Any Tab and other control character in the text will be changed to a space.

  • So when we call prop_add, we need to convert the tab to space.

Fix #26