Following on from the conversation in ipld/ipld#224
this converts this repo to use the latest aegir with Unified CI.

1. Remove all dev deps apart from `aegir`
1. Run the `npx aegir check-project` command to update project config
1. Remove non-Unified-CI github actions
1. Update imports to import from `src/index.js` instead of `@ipld/dag-pb`
1. Rename `test/*.js` to `test/*.spec.js` so aegir can find them
1. Update `tsconfig.json` to extend config from `aegir`
1. Remove `"main"` and other unused fields from `package.json`
1. Use `chai` from `aegir` pre-configured with plugins we use

This will need a follow up PR to `protocol/.github` to add this repo to the
Unified CI [config file](https://github.com/protocol/.github/blob/master/configs/js.json)
so it'll get automated config updates in the future.

Apologies that this PR is so noisy, most of it is from the `check-project` command

…-only