packaging: be more specific about files distributed in packed .tgz

We previously included a few unnecessary files the ui-components .tgz
file. They weren't dangerous, but weren't really appropriate for
distribution. For example:

* .parcel-cache
* .sassrc.json
* .eslint.json
* .yarnrc

Use package.json's 'files' array (an allowlist) instead of .npmignore
(a denylist) to more carefully control what's included.

Besides removing those extra files, CSS and JS source-maps are now
included in the .tgz file distributed by NPM, which should make things
easier for consumers. Since this is an open-source project, there's no
significant drawback to including those maps.

packages/ui-components/package.json

@@ -10,6 +10,12 @@
   "main": "dist/cjs/index.js",
   "module": "dist/esm/index.js",
   "types": "dist/types/index.d.ts",
+  "files": [
+    "dist/",
+    "package.json",
+    "README.md",
+    "LICENSE"
+  ],
   "scripts": {
     "build": "npm-run-all build:typescript build:bundle",
     "build:bundle": "parcel build",