Fix Windows tests, verify in CI (#394)

* ci: Test on Windows too

Adds a Windows test run to CI.
Go setup relies on GHA for this
because Hermit doesn't yet support Windows.

* fix(mapper_windows_test): assert.NotNil => assert.True

assert.NotNil does not exist.

* filecontent mapper: Handle error from directory

If we couldn't read because the source is a directory,
override the original error message.

* fix(test): Handle platform-specific "file not found" messages

go.mod

@@ -1,8 +1,8 @@
 module github.com/alecthomas/kong
 
 require (
-	github.com/alecthomas/assert/v2 v2.1.0
-	github.com/alecthomas/repr v0.1.0
+	github.com/alecthomas/assert/v2 v2.4.1
+	github.com/alecthomas/repr v0.3.0
 )
 
 require github.com/hexops/gotextdiff v1.0.3 // indirect