cross platform package.json using yarpm

[Bebul]

There is an issue for PgnViewerJS mliebelt/pgn-viewer#491
that it was impossible to npm install it because of problems with this chessground repository which used script with infamous $npm_exepath not present on Windows OS:

   "prepare": "$npm_execpath run compile",

I have followed https://stackoverflow.com/questions/53883405/npm-execpath-isnt-recognized-as-an-internal-or-external-command-program-or
which recommended to use yarpm

Then I installed yarpm which added one dependency line and then changed the script line.

So two lines were changed. Dunno, whether the changes in yarn.lock should be included or not...