Improve build sytem support for readline instead of editline

Changes:

- CPP variable is now `USE_READLINE` not `READLINE`

- `configure.ac` supports with new CLI flag

- `package.nix` supports with new configuration option

- `flake.nix` CIs this (along with no markdown)

Remove old Ubuntu 16.04 stop-gap too, as that is now quite old.

Motivation:

- editline does not build for Windows, but readline *should*. (I am
  still working on this in Nixpkgs at this time, however. So there will
  be a follow-up Nix PR removing the windows-only skipping of the
  readline library once I am done.)

- Per
  https://salsa.debian.org/debian/nix/-/blob/master/debian/rules?ref_type=heads#L27
  and #2551, Debian builds Nix with readline. Now we better support and
  CI that build configuration.

This is picking up where #2551 left off, ensuring we test a few more
things not merely have CPP for them.

Co-authored-by: Weijia Wang <9713184+wegank@users.noreply.github.com>

src/libcmd/repl.cc

@@ -5,7 +5,7 @@
 
 #include <setjmp.h>
 
-#ifdef READLINE
+#ifdef USE_READLINE
 #include <readline/history.h>
 #include <readline/readline.h>
 #else
@@ -249,14 +249,14 @@ void NixRepl::mainLoop()
     } catch (SysError & e) {
         logWarning(e.info());
     }
-#ifndef READLINE
+#ifndef USE_READLINE
     el_hist_size = 1000;
 #endif
     read_history(historyFile.c_str());
     auto oldRepl = curRepl;
     curRepl = this;
     Finally restoreRepl([&] { curRepl = oldRepl; });
-#ifndef READLINE
+#ifndef USE_READLINE
     rl_set_complete_func(completionCallback);
     rl_set_list_possib_func(listPossibleCallback);
 #endif