Use an enum for OS Types

[DonJayamanne]

Current we use boolean values to determine whether os is one of Mac, Windows or Linux.
Should consider using an Enum.
This could result in a number of changes, however I believe it is worth, and needs to be done.
Using 3 booleans doesn't work well, when all of them are mutually exclusive.

Suggestion:

export enum OS {
	Linux = 'Linux',
	Mac = 'Mac',
	Windows = 'Windows'
}

@Microsoft/pvsc-team

[brettcannon]

And we will probably eventually want a 4th for WSL

[DonJayamanne]

Should we consider that as an OS? Probably not.
I don't know.

[d3r3kk]

If we can get away with just seeing it as Linux, that would be ideal.

[brettcannon]

@DonJayamanne depends on what we use the enum for. 😉 I mean we won't be able to rely on something being Windows to get the path stuff right, but VS Code will be running on Windows itself, so it's possible we will need it. (I'm not suggesting we start with it now, just something we may need to think about in the future.)

[d3r3kk]

This has been around for some time, see client/common/utils/platform.ts