feat: remove nodejs specific deps from src/core lib API

[SgtPooki]

for filecoin-pin-website, I needed to polyfill process and Buffer. We should ensure we're using ESM browser-compatible structures so users don't need to do that.

    // Optional override: allow selecting a specific provider via env vars
    const envProviderAddress = process.env.PROVIDER_ADDRESS?.trim()
    const envProviderIdRaw = process.env.PROVIDER_ID?.trim()

Also, we should remove EventEmitter from car-blockstore and normalize those so we have a singular API surface area for nodejs and browser users (discussed in comments in #83)