Prevent cmd.exe popup on Windows

[C0Nd3Mnd]

On Windows, a cmd.exe window pops up for a brief moment when the plugin initializes. This PR changes it so the window is hidden.

[predragnikolic]

Hello,

I do not use Windows, so I cannot test this there,
but this PR doesn't work on Linux, as it throws this error:

Traceback (most recent call last):
  File "/home/predragnikolic/.config/sublime-text/Packages/LSP/plugin/core/windows.py", line 249, in start_async
    cannot_start_reason = plugin_class.can_start(self._window, initiating_view, workspace_folders, config)
  File "/home/predragnikolic/.config/sublime-text/Packages/lsp_utils/st3/lsp_utils/npm_client_handler.py", line 110, in can_start
    reason = super().can_start(window, initiating_view, workspace_folders, configuration)
  File "/home/predragnikolic/.config/sublime-text/Packages/lsp_utils/st3/lsp_utils/_client_handler/abstract_plugin.py", line 120, in can_start
    message = cls.is_allowed_to_start(window, initiating_view, workspace_folders, configuration)
  File "/home/predragnikolic/.config/sublime-text/Packages/LSP-volar/plugin.py", line 39, in is_allowed_to_start
    startupinfo = subprocess.STARTUPINFO()
AttributeError: 'module' object has no attribute 'STARTUPINFO'

The reason being:

https://docs.python.org/3/library/subprocess.html#windows-popen-helpers
The STARTUPINFO class and following constants are only available on Windows.

[C0Nd3Mnd]

Thanks for the heads-up. I'll update this PR in the coming days and I'll make sure to test it under Windows and on Linux this time. I don't have a Mac right now so I'm afraid I can't test on macOS for the time being.

[C0Nd3Mnd]

I fixed the issue. Tested it successfully with Windows 10 21H2 and Fedora 35.

[predragnikolic]

Thanks :)

Tested on Linux ✔️ works
I assume that it also works on Mac Os as well,
but I will test it on a Mac till the end of the week just to be sure.
Than I will proceed with merging this.

[predragnikolic]

Thanks again for the PR. :)