[VS Code extension] Automate Nix NPM dependency file generation

[sir4ur0n]

Currently the VS Code extension is an NPM package, i.e. there are package[-lock].json files listing the dependencies.
As we build with Nix, we need a Nix way to convert those NPM files into Nix files.
This is currently done by node2nix.

A limitation of node2nix is that the files must be generated manually and version-controlled, meaning that:

• updating the NPM dependencies always requires a manual step, which is easy to forget
• NPM dependencies update automation via Dependabot opens PRs that fail CI, and require manual update to build

We originally attempted to address this problem by switching to yarn-plugin-nixify in #944 but it turns out this plugin does not seem mature enough for our use case.

There exists other solutions out there, including:

• npmlock2nix
• yarnpnp2nix
• rolling out our own IFD: build a derivation that runs node2nix, stores the output Nix files, then import those files

The balance is tricky to find between:

• automated vs manual
• library vs hand rolling our own solution
• popular/idiomatic vs unpopular/confusing

Each solution (node2nix, npmlock2nix, yarn-plugin-nixify, yarnpnp2nix, custom) seems to only check 2 out of the 3 🤷