Restructure core:terminal for better Windows support

core/terminal/terminal_posix.odin

@@ -1,3 +1,4 @@
+#+private
 #+build linux, darwin, netbsd, openbsd, freebsd, haiku
 package terminal
 
@@ -7,3 +8,9 @@ import "core:sys/posix"
 _is_terminal :: proc(handle: os.Handle) -> bool {
 	return bool(posix.isatty(posix.FD(handle)))
 }
+
+_init_terminal :: proc() {
+	color_depth = get_environment_color()
+}
+
+_fini_terminal :: proc() { }