refactor: inject I/O in fetch-all-bundles.ts, add-proposal.ts

[dckc]

follow-up to #84 to inject I/O

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
packages/synthetic-chain/src/lib/bundles.ts	70% smaller
packages/synthetic-chain/src/cli/chain.ts	39% smaller
scripts/fetch-all-bundles.ts	38% smaller
scripts/add-proposal.ts	32% smaller
packages/synthetic-chain/src/lib/webAsset.js	9% smaller
packages/synthetic-chain/src/cli/proposals.ts	0% smaller
packages/synthetic-chain/test/snapshots/test-exports.ts.md	Unsupported file format
packages/synthetic-chain/test/snapshots/test-exports.ts.snap	Unsupported file format

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[dckc]

adding readdir and basename to the args of makeFileRd would be an incompatible change, so makeDirRd is separate.

[turadg]

I'm approving reluctantly.

The code looks sound. My reservations are:

1. Security that has no value for tests
2. New abstractions that are unfamiliar and increase cognitive overhead
3. More code to maintain
4. One-off abstraction of file system

I would be less reluctant if Endo or some other package (@dckc/ocap?) had a thorough abstraction for FS that could be dropped in. Especially if it rhymes with existing APIs.