Force utf8 encoding when writing vfs temp files

[jneira]

• In windows hPutStr uses by default, the default system encoding to write content
  • In windows almost never is utf8 (in unix and macos is)
• This make that utf8 original content are being writed incorrectly in the vfs temp file
• AFAIK all haskell-lsp clients are assuming that contents must be in utf8
  • f.e. hlint do it
  • if it is no the case we should save, or guess(?), the original file encoding
    • or convert the original encoding to utf8 in clients
• It hopefully closes Lint error: hGetContents: invalid argument (invalid byte sequence) (Win10 + Atom) haskell-ide-engine#1560

[jneira]

The JSON spec says:

JSON text SHALL be encoded in UTF-8, UTF-16, or UTF-32. The default encoding is UTF-8, ...

So being the lsp data format json, afaiu this only would break clients that are using UTF-16 or UTF-32

[cocreature]

Looks reasonable to me. We don’t use the temp files at all in ghcide so this doesn’t affect us directly.