fix: #25 Fixes vim:E976 error when cursor is on a blob.

[rpopic2]

• How to reproduce the original error:
  Error executing lua callback: Vim:E976: using Blob as a String #25
  Open any binary file, move cursor on a blob(blue characters starting with ^), throws vim:E976.
  This error is thrown when blob is provided for function vim.fn.matchstr(), instead of a string.
• What changes I've made:
  • In function matchadd() (line 12),
  • Check whether the current line is a blob, seeing if fn.type(line) is a vim.v.t_blob.
  • If it is a blob, return early from matchadd()
• Test results:
  I tried reproducing the bug as I mentioned above, and it was fixed.
  Testing environment: MacOS Ventura 13.1, Apple M2, nvim v0.8.3

[Forsigg]

Hello! I see that PR approved, but not merged. I also get this error from issue. @alanchenvon please merge that PR if all ok. And thanks @rpopic2 for that fix!