Since HEAD will be our 'starting branch', let's special-case it, so we
can just iterate over BRANCHES without needing to pull out HEAD first.

Technically, HEAD doesn't need to point to a branch, but since we're
only creating a prototype, we won't cover special cases like detached
HEADs.

Once we build the tree in Unity we'll only be interested in branches.
Tags are only used to give specific commits in the tree special names
... or so is our assumption. Technically, tags can not only point to
commits, but we aren't gonna handle those cases since we are only
working on a prototype and tags pointing to commits in branches is the
most common...

Since we no longer track all git objects, OBJECTS is a misnomer.

Spawning multiple git commands is more expensive than running a single
command and then parsing the output in JavaScript.

The previous format was a bit too low-level (by sticking too close to
the native git object format). That would probably make it harder to
work with in Unity. So change the format to be more like the git
porcelain command outputs.

The authors object was being serialized as an array of objects, where
each object had a single key-value pair. This was unnecessary, as the
authors object could be serialized directly as a key-value pair object.

Since the switch from GJS to node the types are no longer needed.

Annotated tags (as opposed to lightweight tags) are actually a type of
git object. They contain a reference to a git object (usually a commit)
and some metadata (tagger, message, ...). We will only track them as if
they were a normal ref though since we don't create about the metadata.

While I am not that familiar with node, the port was relatively easy.
The advantage is that it's more portable and doesn't require the user
to have GJS installed (i. e. you don't need to use linux).

Also, the performance seems to be better with node than with GJS.

Using a Set instead of an Array for commit hashes allows us to avoid
duplicates.

String.prototype.split returns an array with one empty string element,
if the calling string is empty, which may be the case when calling
`"$(git tag)".split('\n')`. This was causing the script to fail when
no git tags exist in the repo.

Add some basic tools to the project like prettier, eslint etc. Nothing
else going on in this commit.