Make better use of GitHub Actions warnings and errors #16

I have a few suggestions for improvements. These are not tested, but should work 😀

Errors
Errors could be directly sent back to Actions by changing entrypoint.ps1 L35 to:

foreach ($i in $errors) {
    Write-Output "::error file=$($i.ScriptName),line=$($i.Line),col=$($i.Column)::$($i.RuleName) - $($i.Message)"
}

Warnings
Warnings could be directly sent back to Actions by changing entrypoint.ps1 L38 to:

foreach ($i in $warnings) {
    echo "::warning file=$($i.ScriptName),line=$($i.Line),col=$($i.Column)::$($i.RuleName) - $($i.Message)"
}

PSSA ParseError
PSSA also outputs "ParseErrors". These currently seem to be ignored by this Action, but could be reported with other errors like by changing entrypoint.ps1 L27 to:

$errors   = $issues.Where({($_.Severity -eq 'Error') -or ($_.Severity -eq 'ParseError')})

Reference: https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#setting-a-warning-message