Add support for JUnit format

[calebcartwright]

ShellCheck does not natively support the JUnit XML format.

Seems the best way to go about adding JUnit support (per ShellCheck docs) is to utilize the checkstyle xml format (i.e. -f checkstyle) and then use an xslt to transform that to JUnit xml format.

[calebcartwright]

Here's some info on how we could go about implementing the solution recommended in the ShellCheck docs:

We would need some additional logic here so that when the user selects JUnit as the format that we will pass checkstyle to shellcheck (otherwise we should just pass whatever format was selected).

Then this line that executes shellcheck can be tweaked to get access to the checkstyle xml output so that we can then perform the transform.

This npm package looks like the best/most lightweight dependency we can incorporate to do the transform.

Here's the XSLT to use:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output encoding="UTF-8" method="xml"></xsl:output>

  <xsl:template match="/">
    <testsuite>
      <xsl:attribute name="tests">
        <xsl:value-of select="count(.//file)" />
      </xsl:attribute>
      <xsl:attribute name="failures">
        <xsl:value-of select="count(.//error)" />
      </xsl:attribute>
      <xsl:for-each select="//checkstyle">
        <xsl:apply-templates />
      </xsl:for-each>
    </testsuite>
  </xsl:template>

  <xsl:template match="file">
    <testcase>
      <xsl:attribute name="classname">
        <xsl:value-of select="@name" />
      </xsl:attribute>
      <xsl:attribute name="name">
        <xsl:value-of select="@name" />
      </xsl:attribute>
      <xsl:apply-templates select="node()" />
    </testcase>
  </xsl:template>

  <xsl:template match="error">
    <failure>
      <xsl:attribute name="type">
        <xsl:value-of select="@source" />
      </xsl:attribute>
      <xsl:text>Line </xsl:text>
      <xsl:value-of select="@line" />
      <xsl:text>: </xsl:text>
      <xsl:value-of select="@message" />
      <xsl:text> See https://www.shellcheck.net/wiki/</xsl:text>
      <xsl:value-of select="substring(@source, '12')" />
    </failure>
  </xsl:template>
</xsl:stylesheet>

Obviously we'd also need to add JUnit in a couple other places, like the OutputFormat Enum, the task.json manifest to expose JUnit as an option, etc.