Pasting from outside will prepend ascii control chars

- Pasting text (using `ctrl+shift+v`) from outside will ___sometimes___ prepend `^[[200~` at the beginning and `~` at the end.
- Only happens when pasting single line of text (multiple lines of text will trigger the paste window and so fat nothing went wrong)

Reproduce the issue by using ctrl+shift+v to paste a single line from outside WindTerm, then compare it with a multiline paste that opens the paste window. Trace the terminal paste-handling entry point and verify that completed single-line pastes no longer prepend ^[[200~ or append ~.

Written by the indexing model from the issue text.