Support vendor-specific manifest.json

[lidel]

Background

As noted in #398 Firefox supports additional WebExtension APIs, but Chrome throws an error if we include them in manifest.json.

TL;DR this PR will make it possible to provide additional features per vendor in a safe way.

Changes

Manifest is Assembled During Build

This change splits manifest into common and vendor-specific files:

add-on/
├── manifest.common.json       # manifest base that is optionally extended for vendor-specific builds
├── manifest.firefox.json
├── manifest.firefox-beta.json
└── manifest.json              # generated during build

Behaviour of npm run build does not change, but there are new commands in bundle:* namespace that enable developer to quickly switch manifest between different targets and (re)create a valid bundle for a specific vendor.

Build Creates Multiple Bundles

Right now we have two targets enabled by default.
A generic one for Chrome and Brave and firefox for Firefox:

build
├── generic
│   └── ipfs_companion-2.2.0.zip
└── firefox
    └── ipfs_companion-2.2.0.zip

[lidel]

I am merging this to unlock #398.

[alanshaw]

Apologies I didn't get a chance to look at this last week. I just had a quick glance and it looks good to me!