feat(emacs-pkgs/notable): Begin implementation by adding note-taking

I'm writing a note taking system because I don't like any of the
existing ones.

This adds a note-taking function which prompts the user for note
content and persists it in a user-configurable directory.

Next steps are roughly:

* listing notes
* editing notes
* archiving notes

At which point I would consider this functional. The idea is to keep
adding incremental improvements that fit my workflows after that, and
if it ends up being useful for anyone else - great.

Change-Id: I7ae0eb4015511ebdbaeff92d8d32502726eb57c4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1981
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

tools/emacs-pkgs/notable/default.nix

@@ -0,0 +1,10 @@
+{ pkgs, ... }:
+
+pkgs.emacsPackages.trivialBuild rec {
+  pname = "notable";
+  version = "1.0";
+  src = ./notable.el;
+  packageRequires = with pkgs.emacsPackages; [
+    dash f ht s
+  ];
+}