feat: add subpath exports for individual layer modules

[jsve]

Closes #221

Summary

Adds a ./layers/* subpath export so each layer module can be imported on its own:

import { GeoArrowPolygonLayer } from "@geoarrow/deck.gl-geoarrow/layers/polygon-layer";

Importing through the package root makes the bundler resolve every layer module before tree-shaking. That includes the ones that import @deck.gl/geo-layers (A5, Geohash, H3 hexagon, S2, trips) and @deck.gl/aggregation-layers (heatmap). With subpaths, an app that only uses, say, the path, polygon, and scatterplot layers never has to resolve those packages.

Changes

• package.json: add ./layers/* pointing at dist/layers/*.js / *.d.ts. The root "." export is unchanged.
• ./layers/text-layer: null keeps the experimental text layer out of the subpaths, so it is still only reachable as _GeoArrowTextLayer from the root.
• README: short section documenting the subpath imports.

Note that a subpath exposes everything its module exports, e.g. defaultProps from path-layer and getPolygonExterior/getMultiPolygonExterior from polygon-layer. Happy to switch to an explicit list of layer entries, or adjust those module exports, if you'd prefer to keep them out of the public surface.

Testing

• pnpm typecheck, pnpm test, and biome ci . pass.
• Packed the package with pnpm pack and bundled it with Vite in a consumer project that has @deck.gl/core and @deck.gl/layers, but not @deck.gl/geo-layers or @deck.gl/aggregation-layers:
  • importing path/polygon/scatterplot from the root fails: failed to resolve import "@deck.gl/geo-layers" from ".../dist/layers/trips-layer.js"
  • importing the same layers from their subpaths bundles without pulling in either package
  • layers/text-layer is rejected by Vite, Node (ERR_PACKAGE_PATH_NOT_EXPORTED) and TypeScript (TS2307)
• TypeScript (moduleResolution: "bundler") resolves the subpath declaration files with full types.

[kylebarron]

I commented on this in #221