Add main and module fields for resolvers without exports support

[veksa]

The package is exports-only, so resolvers that predate exports — webpack 4
(react-scripts 4) in particular — can't find it at all:
Cannot find module: 'alien-signals'.

This adds two fallback fields pointing at files that are already published:

"main": "./cjs/index.cjs",
"module": "./esm/index.mjs",

exports still wins everywhere it's understood, so nothing changes for Node,
webpack 5, Vite or Rollup — verified, along with build, tests and tsslint.

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