Allow multiple specification URLs

[foolip]

No description provided.

[ddbeck]

Oh good my change in #44 has really confused matters.

You'll need something along the lines of this in index.ts

/** Web platform feature */
export interface FeatureData {
    /** Specification URLs */
    spec: spec_url | [spec_url, spec_url, ...spec_url[]];

and a corresponding spec_url type to make it such that npm run generate-schema-defs creates the desired output.

This suggests a follow-up task for me: write some test that fails if regenerating the schema creates a diff.

[ddbeck]

@foolip I pushed some changes. If you're happy with this, then I am.

[foolip]

Thanks @ddbeck, looks good!