Add "ansi" to `unsupported_term`

In my admittedly idiosyncratic setup (I cargo-culted it from somewhere so there's at least one other person doing this, but probably not many), I use emacs shells with `TERM=ansi`, because my emacs configuration supports basic ANSI escapes (color etc) but not fancier things like cursor positioning. In `linenoise` this should be treated as a dumb/unsupported terminal.