Cmdlet example using only PowerShell.

PSClassCmdlet.psm1

# https://seeminglyscience.github.io/powershell/2017/04/13/cmdlet-creation-with-powershell
using namespace System.Management.Automation
using namespace System.Reflection

[Cmdlet([VerbsDiagnostic]::Test, 'Cmdlet')]
class TestCmdletCommand : PSCmdlet {
    [Parameter(ValueFromPipeline)]
    [object]
    $InputObject;

    [void] ProcessRecord () {
        $this.WriteObject($this.InputObject)
    }
}

$cmdletEntry = [Runspaces.SessionStateCmdletEntry]::new(
    <# name:             #> 'Test-Cmdlet',
    <# implementingType: #> [TestCmdletCommand],
    <# helpFileName:     #> $null
)

$internal = $ExecutionContext.SessionState.GetType().
    GetProperty('Internal', [BindingFlags]'Instance, NonPublic').
    GetValue($ExecutionContext.SessionState)

$internal.GetType().InvokeMember(
    <# name:       #> 'AddSessionStateEntry',
    <# invokeAttr: #> [BindingFlags]'InvokeMethod, Instance, NonPublic',
    <# binder:     #> $null,
    <# target:     #> $internal,
    <# args:       #> @(
        <# entry: #> $cmdletEntry
        <# local: #> $true
    )
)
Export-ModuleMember -Cmdlet Test-Cmdlet