Publish code to compute Baseline status for BCD

[foolip]

There's code in https://github.com/ddbeck/strict-browser-compat-data which we should publish as an NPM package for the benefit of @LeoMcA and other consumers.

We'll maintain it in this repo but publish it as a separate NPM package called compute-baseline.

It should export one method with a signature a bit like this:

function computeBaseline({
  bcd, /* required */
  key, /* required for now, could be optional if a compat object is passed */
  browsers, /* default to core browser set */
}) {}

[foolip]

We should return something like:

{
  baseline: "low" | "high" | false,
  baseline_low_date: "string",
  baseline_high_date: "string",
  support: object /* TBD, depends on what the code does */
}

[captainbrosset]

At our Baseline WebDX CG call yesterday, we quickly talked about this, and one point that came up is the fact that consumers will, over time, likely have a wide range of programming language needs. Forcing people to use a JS function might not always work. One way would be to make the algorithm clear, and document it.

Another way could be, and this is me thinking out loud, to just generate all individual feature baseline statuses in our package. After all, this would just be a build step, not something we maintain. Our package would contain the baseline status for all of the logical web feature groups we create and baseline status for all of the indivudal bcd keys.

[foolip]

Simply publishing a bunch of JSON was the first thing that @ddbeck and I considered, and we'll want to do that eventually, but publishing the code has some benefits. In particular, it avoids this package from having to be co-versioned with BCD. If you already depend on BCD (for example in https://github.com/mdn/yari) then you don't need to worry about contradictions between BCD and Baseline data derived from BCD.