Install dns-over-https-l10n via npm?

Perhaps a silly idea, but not sure if it'd be any easier or anything if we added a package.json file to the **mozilla-l10n/dns-over-https-l10n** repo so we could just treat it like any other dependency (although install it via GitHub instead of npm).

https://github.com/jonathanKingston/http-dns/blob/f6c2d9a0e0d84dc761d02ed1c331bab61306b968/scripts/import_l10n.sh#L3-L10

It'd also reduce the scripts/import_l10n.sh file from this:

```sh
./node_modules/pontoon-to-webext/pontoon-to-webext.js --src temp_l10n/locales --dest src/_locales/
```

To possibly just this:

```sh
./node_modules/pontoon-to-webext/pontoon-to-webext.js --src node_modules/dns-over-https-l10n/locales --dest src/_locales/
```

Or, if we were running in the context of an npm script, it may just be:

```js
"postinstall": "pontoon-to-webext --src node_modules/dns-over-https-l10n/locales --dest src/_locales"
```