Fixed #359: Option to use non-global Electron

[davesmith00000]

This PR adds a new configuration option to the Indigo plugin (both Mill and sbt) to change how Electron is installed and invoked, like this (Mill shown here):

// (Pick one...) 
val electronInstall = ElectronInstall.Latest // will install and use version 18.0.0+
val electronInstall = ElectronInstall.Version("^17.0.0") // will install and use 17.0.0+
val electronInstall = ElectronInstall.Global // will use the globally installed version
val electronInstall = ElectronInstall.PathToExecutable(path) // Assumes you know what you're doing...

Defaults to global for two reasons:

1. Doesn't break existing experience
2. Local installs are slower and rather inefficient

Regarding the inefficiency, there might be a future piece of work to make all this significantly more performant, but I decided not to do that yet - if anyone wants it I'm sure an issue can be raised. Essentially if we build the npm deps as part of the build DAG then it can be cached and reused.