Tidy up structure of briefcase

I had a spare fifteen minutes and decided that I should tidy up my
monorepo. The work of tidying up is not finished; this is a small step in the
right direction.

TL;DR
- Created a tools directory
- Created a scratch directory (see README.md for more information)
- Added README.md to third_party
- Renamed delete_dotfile_symlinks -> symlinkManager
- Packaged symlinkManager as an executable symlink-mgr using buildGo

tools/run/README.md

@@ -0,0 +1,30 @@
+# run
+
+Simplify the commands you call to run scripts on the command line.
+
+```shell
+> run path/to/file.py
+> run path/to/file.ts
+```
+
+## How?
+
+Define a run.json configuration mapping commands to filename extensions like
+so:
+```json
+{
+  ".ts": "npx ts-node $file",
+  ".py": "python3 $file"
+}
+```
+
+Then call `run path/to/some/file.ts` on the command line, and `npx ts-node
+file.ts` will run.
+
+## Installation
+
+Install `run` using Nix.
+
+```shell
+> nix-env -iA briefcase.run
+```