chore: use npm 7 workspaces instead of lerna bootstrap

It's faster - running `time npm i` shows a nice speed up:

lerna

```
npm timing command:install Completed in 183859ms
npm timing npm Completed in 184077ms
npm info ok

real	3m4.246s
user	2m48.066s
sys	0m58.948s
```

npm7 + workspaces

```
npm timing command:install Completed in 101724ms
npm timing npm Completed in 101914ms
npm info ok

real	1m42.075s
user	1m52.422s
sys	0m46.022s
```

The downside is it requires npm7 - it's bundled with node 15 but will
require users up upgrade if they are using node 14 or below.

Author: achingbrain

package.json

@@ -3,7 +3,6 @@
   "version": "1.0.0",
   "description": "JS implementation of the IPFS UnixFS",
   "scripts": {
-    "postinstall": "lerna bootstrap",
     "reset": "lerna run --parallel clean && rm -rf packages/*/node_modules node_modules",
     "test": "lerna run --parallel test",
     "test:node": "lerna run --parallel test:node",
@@ -49,5 +48,11 @@
     "Diogo Silva <fsdiogo@gmail.com>",
     "jbenet <juan@benet.ai>",
     "Bernard Mordan <bernard@tableflip.io>"
-  ]
+  ],
+  "workspaces": [
+    "packages/*"
+  ],
+  "engines": {
+    "npm": ">=7.0.0"
+  }
 }