perf: Define top-level earcut worker pool

[kylebarron]

Change list

• Define an earcut worker pool once in global scope, and reuse it for any polygon layers
• This should (significantly?) improve perf for any polygonal layers (though not the h3 layer)
• Load the worker source as text from the version in node_modules. (earcut.worker.min.js is 15KB, and much easier to not have it be async). This means we don't need any async step, nor do we need to keep the worker source version in sync with our geoarrow-js version in package.json

Depends on geoarrow/geoarrow-js#39

Closes #909