When backfill runs, if there are outputFolders that doesn't exist, ignore it

[kenotron]

Currently, the backfill command fails if the resultant command does not generate the outputFolders listed. If we were to share this config with many packages within a monorepo, we would have very different configs for each package... because some may not generate all the output folders.

[kenotron]

Just remembered another scenario: there are times that we would allow different args to be passed to a npm run build that would generate ESM folder vs COMMONJS folder. So, backfill.config.js should allow us to specify both as outputFolder and just ignore the unused outputFolder value.