websharper-html template is not cross-platform

[gbtb]

Hello 👋

This line obviously would work only on Windows.

templates/WebSharper.Templates/templates/Offline-FSharp/WebSharper.Offline.FSharp.fsproj.in

Line 6 in 2d932ae

<StartProgram>$([System.Environment]::GetEnvironmentVariable(`WinDir`))\explorer.exe</StartProgram>

I'd like to suggest adding separate blocks for Linux and maybe macOS. I can't tell about macOS, but for Linux there is no single file manager that is guaranteed to be present. However, one can use xdg-open instead, which in normal conditions should open a default program user or distro creator chose for handling mime type of filepath. Again, one cannot be 100% sure that every distro have xdg-open installed, but most graphical distros should. In my case, it successfully opens my default file manager Dolphin.