Add JavaScript and TypeScript kernels

[thomasjm]

Brings back a JavaScript/Node.js kernel and adds a TypeScript one, both built on tslab (Node 24,
zeromq 6). The old ijavascript setup isn't a usable starting point: nodePackages and node2nix
are both gone from nixpkgs 26.05, so the kernel and its npm dependencies are packaged here.

• Kernels. tslab registers both from one binary (--js or not), so they share the package
  set, node_modules and language server. The zeromq native addon is compiled from source
  against Nixpkgs' libzmq, since upstream's cmake-ts/vcpkg build needs the network. Node REPL,
  typescript-language-server, and a sample environment each.
• Subpackages. A checked-in package-lock.json is the pin; importNpmLock fetches each
  tarball by its integrity hash, and a closure walk builds a node_modules for just the
  selected packages. 55 curated libraries: d3, vega/vega-lite, echarts, plotly, Observable Plot,
  arquero, apache-arrow, mathjs, the ml.js family, turf, sql.js, and so on. Install scripts are
  not run; update.sh rejects only those that genuinely compile something.
• D3. Selecting d3 adds a codedown-d3 helper with render (browser-side, interactive)
  and renderStatic (jsdom, SVG in the kernel).

The kernel's working directory is the user's notebook directory, so packages reach the kernel
through NODE_PATH, tslab's compiler is pointed at the tree it lives in, and a small stdio proxy
seeds the language server's workspace with a jsconfig.json or tsconfig.json. Without that
last part the server resolves nothing: hovers come back any and completion falls back to text
matches.

Tests cover both kernels, package selection and isolation, both D3 paths, and LSP
completions/hovers against an environment package.

Still to do: regenerating the feature matrix and options docs, and removing
old_languages/javascript.