New dependency on NPM

[rbergen]

We will soon merge an update to the on-board web UI (PR #431 to be precise) which will introduce a dependency on NPM for the on-onboard website build. That's a build that is executed as a part of all NightDriverStrip PlatformIO project builds, meaning that everyone who builds locally will have to have NPM installed.
If NPM is not available on a machine a NightDriverStrip build is attempted on, an error will be shown that indicates that NPM is missing and needs to be installed.

NPM is available on all platforms supported by PlatformIO, and instructions to install it are available here:
https://docs.npmjs.com/downloading-and-installing-node-js-and-npm.
That documentation recommends the use of a Node version manager. Those seeking a "shortcut" to install a recent version of Node.js and NPM can get an installer from https://nodejs.org/en/download, instead.

Many people working on a UN*X-based development machine will probably find NPM is already installed on it.

We understand adding the NPM dependency introduces one more step towards getting going with (the source code of) NightDriverStrip. Considering the ambitions we have towards the future developments of the web UI - as indicated in part by the work items in the Issues list - we feel this is a necessary thing to do.

Cheers,
Rutger