hack: enable builds on non-Windows

v2/internal/ffenestri/windows/x64/x64.go

@@ -1,3 +1,5 @@
+// +build windows
+
 package x64
 
 import _ "embed"

v2/internal/ffenestri/windows/x64/x64_stub.go

@@ -0,0 +1,9 @@
+// +build !windows
+
+// This is a stub to define the following even though they don't exist
+// on non-Windows systems.  Note: This is not the right way to handle this.
+package x64
+
+var WebView2 []byte
+
+var WebView2Loader []byte