Add UTF-8 BOM for PowerShell files

[xtqqczze]

As discussed in #100, Windows PowerShell defaults to the Windows-1252 file encoding, so the UTF-8 BOM may be needed to avoid encoding issues in PowerShell files.

https://docs.microsoft.com/powershell/scripting/dev-cross-plat/vscode/understanding-file-encoding

[iSazonov]

Windows PowerShell defaults to the Windows-1252 file encoding

On English Windows version. What about localized Windows version?

[xtqqczze]

Windows PowerShell defaults to the Windows-1252 file encoding

On English Windows version. What about localized Windows version?

Yes, it may differ. That's one more reason to add a UTF-8 BOM to files interpreted by Windows PowerShell. 😄