Use a null package for string_decoder

[fearthecowboy]

Fixing CG for string_decoder:

The only purpose for string_decoder was to support old versions of node, and isn't actually needed/used for modern versions of node/electron at all.

Unfortunately, you can't delete a dependency using overrides in the package.json file -- you can only point to a different version (or directly to a package file)

I created a string_decoder 10.0.0 package file, and add that to the overrides, and it appears that everything works.

The trouble is, that the path to the string_decoder-10.0.0.tgz has to be absolute (either on disk, or a URL to a location on the web)

I don't want a whole repo to create a string_decoder placeholder, so I just created a simple package and put it in the assets of the latest pre-release and took the direct-link to that and put it into the package.json overrides section:

 "string_decoder": "https://github.com/microsoft/vscode-arduino/releases/download/v0.4.12-rc2/string_decoder-10.0.0.tgz"

and then removed the package-lock files and did an NPM install and it works, and the vscode tests pass