Display inline graphics in Sage REPL if running in iTerm2 terminal on macOS

[mkoeppe]

This ticket is to display inline grpahics in the Sage REPL
if it is running in an iTerm2 terminal on macOS.

• iTerm2: https://www.iterm2.com/
• Python interface: https://github.com/asmeurer/iterm2-tools
• Detect whether iTerm2 is running (should work across ssh):
  https://raw.githubusercontent.com/gnachman/iTerm2/master/tests/isiterm2.sh

Related ticket:

• add SIXEL support for terminal display #26544: add SIXEL support for terminal display

CC: @slel

Component: user interface

Keywords: repl, inline, macOS, terminal

Issue created by migration from https://trac.sagemath.org/ticket/21040

[slel]

Changed keywords from none to repl, inline, macOS, terminal

[mkoeppe]

closing for the same reason as #26544