Add io_streams_t::out_is_terminal()

This encapsulates a "is our output going to the terminal" check we do
in a few places - functions, type, set_color, possibly test

fish-rust/src/builtins/shared.rs

@@ -7,6 +7,9 @@
 use crate::wgetopt::{wgetopter_t, wopt, woption, woption_argument_t};
 use cxx::{type_id, ExternType};
 use libc::c_int;
+use libc::isatty;
+use libc::STDOUT_FILENO;
+
 use std::borrow::Cow;
 use std::fs::File;
 use std::io::{BufRead, BufReader, Read};
@@ -164,6 +167,10 @@ pub fn ffi_ref(&self) -> &builtins_ffi::io_streams_t {
         unsafe { &*self.streams }
     }
 
+    pub fn out_is_terminal(&self) -> bool {
+        !self.out_is_redirected && unsafe { isatty(STDOUT_FILENO) == 1 }
+    }
+
     pub fn stdin_is_directly_redirected(&self) -> bool {
         self.ffi_ref().ffi_stdin_is_directly_redirected()
     }