Add nodejs: WebAssembly Node package with no native dependency

New nodejs/ package: a wasm-bindgen crate (vtracer-wasm) built with wasm-pack
that wraps the vtracer framework, plus a thin JS layer for file I/O. Image
decoding (png/jpeg/gif/bmp via the image crate) runs in wasm too, so the
package has zero native dependencies — no sharp, no node-gyp.

No separate general-purpose wasm crate: the Node package directly owns and
wraps the wasm. Excluded from the cargo workspace (wasm-bindgen cdylib), built
with wasm-pack.

JS API (camelCase options): convertBuffer, convertPixels, convertFile,
convertFileSync — each taking an Options object (preset, colorMode,
hierarchical/cutout mosaic, mode, palette, maxColors, optimize, ...). Ships
index.d.ts types and a node smoke test. README updated.

Verified: builds to wasm32-unknown-unknown; `node test.js` passes; output
matches the CLI/Python bindings (253 paths on the tank sample).

nodejs/.gitignore

@@ -0,0 +1,4 @@
+/pkg
+/target
+/node_modules
+Cargo.lock