`Object.source` property & `Source` class

To be released.

 -  Added `source` property to `Object` class in Activity Vocabulary API.

    [[#114]]

     -  Added `Object.source` property.

     -  `new Object()` constructor now accepts `source` option.

     -  `Object.clone()` method now accepts `source` option.

 -  Added `Source` class to Activity Vocabulary API.  [[#114]]

 -  Improved the performance of `Object.toJsonLd()` method.

     -  `Object.toJsonLd()` method no longer guarantees that the returned

    Bun, and fills the *deno.json* file with the default `compilerOptions` on

    Deno.

[#114]: https://github.com/dahlia/fedify/issues/114

Version 0.13.0

--------------

  range:

  - "http://www.w3.org/2001/XMLSchema#boolean"

- singularName: source

  functional: true

  compactName: source

  uri: "https://www.w3.org/ns/activitystreams#source"

  description: |

    The `source` property is intended to convey some sort of source from which

    the `content` markup was derived, as a form of provenance, or to support

    future editing by clients.  In general, clients do the conversion from

    `source` to `content`, not the other way around. 

  untyped: true

  range:

  - "https://www.w3.org/ns/activitystreams#Source"

- singularName: proof

  pluralName: proofs

  singularAccessor: true