Add module for aerc (#3070)

This adds support for configuring email accounts, with automatic smtp, imap,
sendmail (msmpt) and maildir (mbsync, offlineimap) setup in aerc,
via `accounts.email`.

tests/modules/programs/aerc/extraConfig.expected

@@ -0,0 +1,18 @@
+# Generated by Home Manager.
+
+[general]
+unsafe-accounts-conf = true
+
+[ui]
+index-format = null
+mouse-enabled = false
+sidebar-width = 42
+sort = -r date
+spinner = true,2,3.400000,5
+test-float = 1337.420000
+
+[ui:account=Test]
+sidebar-width = 1337
+
+[ui:account=c_imap-passcmd-notls-nostarttls-extraConfig]
+index-format = %42.1337n