fix: more stable packaging

[smorimoto]

Closes #330

This PR updates tsdown configuration to use tsdown's Auto-Generating Package Exports to support more stable packaging. In addition, we support dual ESM and CJS packages. To achieve this, we've moved away from the node and browser platform configuration and instead use a single neutral platform, marking the node-related dependencies as external dependencies so that we don't have two duplicate tsdown configurations. There is no impact at runtime.

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[andriiholovan]

@bgub please check on this

[bgub]

@smorimoto @andriiholovan I appreciate the PR, but I have a few concerns.

I like the simplicity of merging configurations into one, but I feel that it "undoes" the purpose of Eta v4. The only change in Eta v4 compared to v3 is that it's ESM-only, because I'm trying to help the ecosystem move forward.

Additionally, making a CJS version of core is probably unnecessary, and TSDown automatically uses the node platform for cjs builds, so neutral won't help.