Split up eco.json and add tooling

[JeffML]

Consider splitting eco.json into smaller files by ECO category: ecoA.json, ecoB.json,..ecoE.json; this would make these files easier to read in an editor.

Add "to:" field in JSON openings linking one variation to another.

Add 'tooling' folder for stand-alone Node JavaScript files (.mjs extension), including:

1. how interpolated.json is generated
2. how to generate "from:" data from "to:" data
   1. (would include transitions from other openings)

[JeffML]

Fixed by #15