feat(tileset): Use Web Workers

[donmccurdy]

Work in progress. Open questions:

• Avoid starting Web Workers until a widget request is made. We don't want to spawn workers if the user is just rendering the tileset.
• How many Web Workers should be spawned? A singleton? One per dataset? A pool of N workers, with datasets balanced among them?
• If we're using ES Module workers, is that compatible with CJS builds? Workers will be ESM-only.
• If workers are opt-in or opt-out, what's the API to switch? Opt-out, by widgetSourceWorker: false option.
• If Microbundle generates named chunks like widget-tileset-worker-BXxfh1Ze.js, is that compatible with package.json#exports? Replaced microbundle with tsup.

These issues could probably be solved like deck.gl does, by publishing worker code to a CDN and loading that... But this makes builds and development more complicated, and CDNs may fail, I'd prefer to avoid the CDN.

The last version of Node.js that doesn't support require(esm) will reach end-of-life in April... maybe we can say Web Workers are available only for ESM builds? We can use an extra entrypoint (@carto/api-client/worker) for the worker code.