feat: add functions level to metadata object

[eduardoboucas]

Follow up to #122 and #126, which haven't been released yet (see #125). It changes the shape of the metadata object to have function-specific metadata under a functions key.

Before:

export const metadata = {
  func1: {
    url: "file:///some/path/func1.ts"
  },
  func2: {
    url: "file:///some/path/func2.ts"
  }
}

After:

export const metadata = {
  functions: {
    func1: {
      url: "file:///some/path/func1.ts"
    },
    func2: {
      url: "file:///some/path/func2.ts"
    }
  }
}

This will allow us to more easily export additional metadata that is not tied to a specific function (e.g. the path of the stage 2 file).