Settings editor for enums should handle `undefined` for `default`

[bpasero]

//cc @rzhao271 maybe this is a bug for the settings editor: if a setting is a enum without a default, shouldn't the dropdown indicate this by not showing the setting?

Originally posted by @bpasero in #128345 (comment)

[rzhao271]

It turns out there are quite a few settings that fall under this category:

editor.gotoLocation.alternativeImplementationCommand
editor.gotoLocation.alternativeReferenceCommand
editor.defaultFormatter
editor.gotoLocation.alternativeImplementationCommand
editor.gotoLocation.alternativeReferenceCommand
editor.defaultFormatter
typescript.locale
terminal.integrated.defaultProfile.windows
git.autofetch
gitlens.fileAnnotations.command
gitlens.advanced.commitOrdering
jupyter.jupyterServerType

What I can do is create a default stand-in setting in this case.

[roblourens]

I don't understand, either the setting should be of type string|undefined, or the empty string should be part of the enum. Seems like the registration is wrong otherwise.

[rzhao271]

Turns out my filtering is wrong. I'll see what the other settings are doing. It seems jupyter.jupyterServerType might be within the scope of this issue, though.

[rzhao271]

jupyter.jupyterServerType is the only other setting within the scope of this issue. It has an enum prop declared, but no default prop (the default prop is implicitly undefined).

https://github.com/microsoft/vscode-jupyter/blob/33af7d138cd9763a684c735fcccb6fd35c811a8d/package.json#L1592

The explorer.copyRelativePathSeparator setting has been fixed separately, and the other settings declare default values, even if those default values are empty strings.

I can push in a fix that adds the default value to the options to select from, assuming that default value isn't currently one of the valid enum options, and that it is undefined. One advantage is that instead of having to ping extension authors who might forget to leave out the value, we can add in the fix on our side. But, that scenario brings up the following question:

Why can I create the following setting without a warning, and actually set "emmet.hmm": "c" in the OSS setting JSON file? @aeschli

"emmet.hmm": {
  "type": "string",
  "enum": ["a", "b"],
  "default": "c"
}

[roblourens]

I think that an empty item in a dropdown looks weird, but it's better than picking an arbitrary item when we don't know the default. We should fix this in any core settings or first party extension settings (like jupyter)