build(npm): add license field

This change adds the `license` field to our `package.json` file, which
allows some license checking tools to analyze our dependencies for
license incompatibilities.

The current state of our licensed dependencies is as follows:

```
$ license-checker --summary
├─ MIT: 198
├─ ISC: 31
├─ Apache-2.0: 5
├─ (MIT OR CC0-1.0): 3
├─ BSD-3-Clause: 2
├─ BSD-2-Clause: 2
├─ 0BSD: 2
├─ (MIT OR Apache-2.0): 1
├─ CC-BY-3.0: 1
└─ CC0-1.0: 1
```

Change-Id: I7b9dae1c9f732c7f7491b5d6557dd193b894d70c
Signed-off-by: Chris Kay <chris.kay@arm.com>

package-lock.json

@@ -5,6 +5,7 @@
   "packages": {
     "": {
       "hasInstallScript": true,
+      "license": "BSD-3-Clause",
       "devDependencies": {
         "@commitlint/cli": "^14.1.0",
         "@commitlint/config-conventional": "^14.1.0",

package.json

@@ -1,4 +1,5 @@
 {
+  "license": "BSD-3-Clause",
   "private": true,
   "scripts": {
     "postinstall": "husky install"