v3.0.0

Changes:

• Use fetch to service requests, drop use of d3-request and XMLHttpRequest (XHR). Note that, while similar in many respects, fetch has a different default behavior than XHR. For example, fetch is more secure by default and does not share cookies or credentials unless configured to do so.
• Change loader options format. Clients should use the options.http object property to provide fetch parameters. The options.headers property is no longer supported, instead use options.http.headers.
• The loader http method now directly accepts an http parameter object rather than the top-level loader options. The loader's options.http property is used as the base set of parameters, which may then be overridden by the options argument to the http method.
• Update dependencies.