refactor!(style): disambiguate format property

[bdarcus]

Change format property to templates.

Also, make separate properties for called and inline.

Close: #116

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May also want to distinguish templates and namedTemplates (or templateCollection)?

PS - hard to sort out how to adjust the model to do this! I may have to use types, rather than interfaces, for the templates; something like:

type Context = 
  | Options
  | Template

type Options = {
  options: string;
}

type Template = InlineTemplate | CalledTemplate;

// instead of extends, use &
type RenderList = Options & Template;

type InlineTemplate = {
  template: "InlineTemplate";
}

type CalledTemplate = {
  templateKey: "CalledTemplate";
}

const template1: Context = { options: "test", template: "InlineTemplate" }; // valid
const template2: Context = { options: "test", templateKey: "CalledTemplate" }; // valid
const template1: Context = { options: "test", templateKey: "InlineTemplate" }; // not valid

I don't think you can do that with interfaces.

Unfortunately, in fixing this here, I triggered this long-open bug in the schema-generator:

https://github.com/vega/ts-json-schema-generator/issues/192

Bigger change, while I'm at it, is split style.ts into multiple.