Splice ./universe directory into ./

Manually merging:
- README.md: I added the description from universe/README.md into the heading of
  dotfiles/README.md.
- .envrc: dotfiles/.envrc was a superset of universe/.envrc
- .gitignore: Adding some of the ignored patterns from universe/.gitignore to
  dotfiles/.gitignore

Everything else here should be a simple rename.

universe/third_party/lisp/let-plus.nix

@@ -1,23 +0,0 @@
-{
-  depot ? import <depot> {},
-  universe ? import <universe> {},
-  ...
-}:
-
-let
-  src = builtins.fetchGit {
-    url = "https://github.com/tpapp/let-plus.git";
-    rev = "7cf18b29ed0fe9c667a9a6a101b08ab9661a59e9";
-  };
-in depot.nix.buildLisp.library {
-  name = "let-plus";
-  deps = [
-    depot.third_party.lisp.alexandria
-    universe.third_party.lisp.anaphora
-  ];
-  srcs = [
-    "${src}/package.lisp"
-    "${src}/let-plus.lisp"
-    "${src}/extensions.lisp"
-  ];
-}