Exclude ParentProcessWatcher by default

[ralish]

Disables the parent process watcher for lsp4xml by default.

For additional context on this change see: eclipse-lemminx/lemminx#328.

The support in lsp4xml is introduced in: eclipse-lemminx/lemminx#330.

This change is effectively mirroring that introduced to vscode-java by: redhat-developer/vscode-java#843.

[NikolasKomonen]

I think we need to set defaults specific to the OS. I've tested on Fedora and the server does not end if the client does not close properly.

I've tested on Windows and it looks like the PPW isn't necessary there.

MacOS hasn't been tested yet.

@fbricon would you be able to test MacOS when you have time?

[fbricon]

@NikolasKomonen how did you check on Fedora. @snjeza said should couldn't get zombie processes on linux. It was fine on Mac, as far as I tested

[fbricon]

@ralish can you please sign-off your commit? (git commit -s)

[NikolasKomonen]

@fbricon

To recreate it I:

0. I launched VSCode with the XML LS as either a VSIX or a Launched Extension (from development) with the flag -DwatchParentProcess=\"false\" added to vmargs.
1. I ran jps to see if the ...uber.jar existed.
2. I ran pkill code to kill all instances of VSCode
3. I waited a bit then ran jps, vscode-java disappeared but vscode-xml was still there.
   I already had vscode-java installed, so that is why it showed in jps. But they use the same PPW mechanic so I know it works when enabled.

[fbricon]

@ralish please do the same thing as in https://github.com/redhat-developer/vscode-java/pull/847/files#diff-d106f0e1914f2f6cbf79404a7f0c42b4R68

[ralish]

@fbricon: Amended commit per vscode-java implementation and signed as requested.

[fbricon]

Thanks @ralish!