Plugins can't be executed without extension on Windows

[mvinkler]

Bug report

kn 0.13.1 on Windows is not able to execute plugin without specifying the extension:

PS C:\Users\hudson> kn version
Version:      v0.13.1
Build Date:   2020-03-26 15:54:06
Git Revision: ff2a302
Supported APIs:
* Serving
  - serving.knative.dev/v1 (knative-serving v0.13.0)
* Eventing
  - sources.eventing.knative.dev/v1alpha1 (knative-eventing v0.13.1)
  - sources.eventing.knative.dev/v1alpha2 (knative-eventing v0.13.1)
  - eventing.knative.dev/v1alpha1 (knative-eventing v0.13.1)
PS C:\Users\hudson> kn plugin list
C:\Users\hudson.DESKTOP-T61GB27\AppData\Roaming\kn\plugins\kn-hello.bat
PS C:\Users\hudson> kn hello
Error: unknown command 'hello'
Run 'kn --help' for usage.
PS C:\Users\hudson> kn hello.bat
Hello Knative, I'm a Kn plugin
  My plugin file is C:\Users\hudson.DESKTOP-T61GB27\AppData\Roaming\kn\plugins\kn-hello.bat
  I received arguments: C:\Users\hudson.DESKTOP-T61GB27\AppData\Roaming\kn\plugins\kn-hello.bat

Expected behavior

kn hello executes kn-hello.bat plugin

Steps to reproduce the problem

1. create kn-hello.bat file in ~\AppData\Roaming\kn\plugins\

@echo off
echo Hello Knative, I'm a Kn plugin
echo   My plugin file is %0
echo   I received arguments: %1 %2 %3 %4

2. run kn hello

kn version

PS C:\Users\hudson> kn version
Version:      v0.13.1
Build Date:   2020-03-26 15:54:06
Git Revision: ff2a302
Supported APIs:
* Serving
  - serving.knative.dev/v1 (knative-serving v0.13.0)
* Eventing
  - sources.eventing.knative.dev/v1alpha1 (knative-eventing v0.13.1)
  - sources.eventing.knative.dev/v1alpha2 (knative-eventing v0.13.1)
  - eventing.knative.dev/v1alpha1 (knative-eventing v0.13.1)