Support a proper junit reporter

## Feature

Add a JUnit reporter and allow saving the output to a file.

## Motivation

JUnit reporting format is a standard used by many CI/CD tools, like gitlab, CircleCI, etc.

If you do CI/CD, JUnit reporting is great to improve user experience while maintaining software packages and related tests.

In addition, NodeJS test runner supports very well out-of-the-box the junit reporter and concurrency at the same time. Since ava lacks proper support for junit reporting, it risks to become outdated and lose effectiveness.

## The problem

Ava does not support JUnit reporter.

Even though it supports tap reporters, and you can use tap parser to transform the tap output into a junit reporter, however ava does not provide full information to the tap reporter and therefore does not allow to build a proper junit report.

As example, time duration is missing.

There is an [issue open](https://github.com/avajs/ava/issues/1668) to add duration, however doing it only on `--serial` would be not enough to build a proper junit report.

Start by reviewing AVA's existing TAP reporter support and the related duration issue #1668. Determine how a JUnit reporter should preserve test duration and concurrent results, and how its output should be saved to a file. Done means AVA can produce a proper JUnit report with the required information while retaining concurrency support.

Written by the indexing model from the issue text.