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>

package.nix

@@ -13,6 +13,7 @@
 , changelog-d
 , curl
 , editline
+, readline
 , fileset
 , flex
 , git
@@ -71,6 +72,14 @@
 # Whether to enable Markdown rendering in the Nix binary.
 , enableMarkdown ? !stdenv.hostPlatform.isWindows
 
+# Which interactive line editor library to use for Nix's repl.
+#
+# Currently supported choices are:
+#
+# - editline (default)
+# - readline
+, readlineFlavor ? if stdenv.hostPlatform.isWindows then "readline" else "editline"
+
 # Whether to compile `rl-next.md`, the release notes for the next
 # not-yet-released version of Nix in the manul, from the individual
 # change log entries in the directory.
@@ -219,7 +228,7 @@ in {
     sqlite
     xz
   ] ++ lib.optionals (!stdenv.hostPlatform.isWindows) [
-    editline
+    ({ inherit readline editline; }.${readlineFlavor})
   ] ++ lib.optionals enableMarkdown [
     lowdown
   ] ++ lib.optionals buildUnitTests [
@@ -279,6 +288,7 @@ in {
     (lib.enableFeature enableManual "doc-gen")
     (lib.enableFeature enableMarkdown "markdown")
     (lib.enableFeature installUnitTests "install-unit-tests")
+    (lib.withFeatureAs true "readline-flavor" readlineFlavor)
   ] ++ lib.optionals (!forDevShell) [
     "--sysconfdir=/etc"
   ] ++ lib.optionals installUnitTests [