Support multiple major.minor versions

[RyanCavanaugh]

We should allow folder structures like this:

/jquery
  index.d.ts
  tsconfig.json
  jquery-tests.ts
  /v2
    index.d.ts
    tsconfig.json
    jquery-v2-tests.ts
  /v3.1
    index.d.ts
    tsconfig.json
    jquery-v3.1-tests.ts

This should parse the major and optional minor version number from the folder name and generate versioned packages published under the same name as the parent folder.

[blakeembrey]

This come up in a Gitter chat, but instead of putting meaning onto the directory it might be useful to use semantic folder names and embed the version meaning in package.json. For instance, to support multiple TypeScript versions alongside multiple package versions. To keep it simpler, I'd suggest always nesting or always having it flat - so either multiple versions or not, and not a mix, as mixing it will make it very tricky to keep track of in the case where you're writing .d.ts files for nested files.

So, in this case:

/jquery
  index.d.ts
  tsconfig.json
  jquery-tests.ts

Or:

/jquery
  /v2
    index.d.ts
    tsconfig.json
    jquery-tests.ts
  /v3.1
    index.d.ts
    tsconfig.json
    jquery-tests.ts

Where v2 and v3.1 are not used for the version but are instead only semantic. The actual version is stored in package.json so it could also have a typescriptVersion and other matrix-release related properties.