Move all platform-specific functions in index-*.ts files

[tomaka]

Close #2516
After trying #2516, I had a eureka moment about how to do this properly.

This PR removes the compat module. Instead, all platform-specific functions are now implemented in the main file (formerly index.ts) and injected through the public API of client.ts.

There are now two index.ts files: index-browser.ts and index-nodejs.ts. The package.json file defaults to index-browser.ts, but now has a exports -> node field that instead points to index-nodejs.ts.

In a future PR, I will add a index-deno.ts file that will be the Deno entry point.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

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[wirednkod]

lgtm