chore(package) Use a "files" list

We have a fairly small number of things that we wish to ship in our
package, and they're all nicely arranged in a small number of folders.

Instead of having a sprawling .npmignore file, we should just get with
the recommended best practice, and use a "files" list in package.json.

PR-URL: #3365
Credit: @isaacs
Close: #3365
Reviewed-by: @wraithgar

Author: isaacs

.npmignore

@@ -5,6 +5,13 @@
   "workspaces": [
     "docs"
   ],
+  "files": [
+    "bin",
+    "docs/content/**/*.md",
+    "docs/output/**/*.html",
+    "lib",
+    "man"
+  ],
   "keywords": [
     "install",
     "modules",