Move language to top-level attr on session

[benjreinhart]

I'm working towards a better type for a srcbook object (currently called session):

export type SessionType = {
  id: string;
  dir: string;
  cells: CellType[];
  language: CodeLanguageType;
  'package.json': Record<string, any>;
  'tsconfig.json'?: Record<string, any>;
  openedAt: number;
};

This PR moves language top-level and gets rid of 'metadata' which seems unnecessary with the exception of the header in .src.md. This came about because I'm trying to make tsconfig.json and package.json behave consistently (eventually) and I started thinking I'd put it in the metadata but then they really aren't metadata. However, tsconfig might be put in the "metadata" header in .src.md. So my thinking is to put everything we have today top-level in the session object and only treat the header in .src.md as "metadata".