Split Pikelet up into subcrates and add VS Code extension

[brendanzab]

This begins our work on #97.

As part of this change, based on what I've been seeing in Gluon, Argon, Lark, and rust-analyzer, I've begun to break up Pikelet into separate crates. The new directory structure looks like this:

.
├── assets
├── book
├── crates
│   ├── pikelet
│   ├── pikelet-driver
│   ├── pikelet-elaborate
│   ├── pikelet-language-server
│   ├── pikelet-library
│   ├── pikelet-repl
│   └── pikelet-syntax
├── editors
│   └── code
└── tools

This should give us the following benefits:

• faster RLS times when making changes
• makes it easier for newcomers to focus on a single crate - say, if they are interested in a specific thing, eg. editors, type checking, compilation, parsing, etc.
• enforces a DAG between some of our components, which is nice architecturally
• reduces the need for cargo features, making it easier just to grab the driver for embedding the compiler on a web page, for instance

The downsides are:

• we end up with some stubby little crates
• there's more jumping between directories
• harder to make use of dead-code warnings

I've made a subdirectory for editors, as it will give us some incentive to add some more, on top of just VS Code!

[brendanzab]

Sooo, the plugin keeps crashing, but oh well! Thinking I'll just merge this for now. It's a bit of a shake up, so it'd be nice to do this sooner rather than later.