refactor(handlers): refactor to plugins [BREAKING CHANGE]

[chimurai]

Description

• refactor src/_handlers.ts into separate plugins which are loaded by default.
  • debugProxyErrorsPlugin: subscribe and debug http-proxy error events
  • proxyEventsPlugin: subscribe to proxy events via the on-option
  • errorResponsePlugin: error response on proxy error
  • loggerPlugin: output events to logger. default: console
• refactor htt-proxy events:

  // before:
  createProxyMiddleware({
    onProxyRes: () => {}
  });
  
  // after:
  createProxyMiddleware({
    on: {
      proxyRes: () => {}
    }
  });

• SSE close events do not make it through proxy running on Windows #678 (unconfirmed)
• hpm 1.0.6 Erroring for sockets when target server is restarted  #476 (unconfirmed)

Motivation and Context

Simplify internals and allow more customisation in the future

How has this been tested?

• unit-tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

Coverage decreased (-1.9%) to 96.944% when pulling e870e81 on refactor-error-handling into 7341704 on v3.