Add Junit reporter

[sirbrillig]

This adds a new reporter type, junit which can be used to output JUnit XML format.

Fixes #106

There seem to be slightly different definitions for the schema of this format.

• https://github.com/testmoapp/junitxml
• https://www.ibm.com/docs/en/developer-for-zos/17.0.x?topic=formats-junit-xml-format
• https://github.com/windyroad/JUnit-Schema/blob/master/JUnit.xsd
• https://theembeddedkit.io/blog/junit-xml/

Notably, do we need an id attribute or a time attribute? I believe that id is not needed but time might be good so I've added support for that.

Here's an example of the one used by this PR:

<?xml version="1.0" encoding="UTF-8"?>
<testsuites tests="1" failures="1" errors="0" time="0.000">
        <testsuite name="bin/store/code/review-stuck-orders.php" tests="1" failures="1" errors="0" time="0.000">
                <testcase name="line 20, column 5" classname="ImportDetection.Imports.RequireImports.Import" time="0">
                        <failure type="ImportDetection.Imports.RequireImports.Import" message="Found unused symbol &apos;Foobar&apos;.">Line 20, Column 5: Found unused symbol &apos;Foobar&apos;. (Severity: 5)</failure>
                </testcase>
        </testsuite>
</testsuites>