Default presentation of Explorer: Copy Relative Path Separator in Setting editor is confusing

[IllusionMH]

Issue Type: Bug

Would be noticeable on Windows because of (implicit) default \\.

Steps to reproduce:
0. Make sure you are using Windows and reset explorer.copyRelativePathSeparator to default value.

1. Open Settings view and search for explorer.copyRelativePathSeparator - dropdown has / displayed (and setting not changed - no blue border).
   

2. Perform Copy Relative Path - path copied with \\ in it (while dropdown displays /)

3. Change setting to \\ and then "back" to / - setting now marked as changed
   

4. Perform Copy Relative Path - path copied with / in it as expected

I think it's because this setting doesn't have default value and dropdown will display first from list, however implementation has different logic.

VS Code version: Code - Insiders 1.59.0-insider (87d692b, 2021-07-09T05:13:26.890Z)
OS version: Windows_NT x64 10.0.19041
Restricted Mode: No

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz (12 x 3192)
GPU Status	2d_canvas: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on oop_rasterization: enabled opengl: enabled_on rasterization: enabled skia_renderer: enabled_on video_decode: enabled vulkan: disabled_off webgl: enabled webgl2: enabled
Load (avg)	undefined
Memory (System)	31.72GB (15.15GB free)
Process Argv	--crash-reporter-id cf52dbde-2c94-45c3-9bf3-45d8b9966b04
Screen Reader	no
VM	0%

Extensions: none

A/B Experiments

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pythontb:30258533
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vsdfh931cf:30280410
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pythondataviewer:30285072
vscus158:30321503
pythonvsuse255:30319630
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vsccppwt:30312692
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[bpasero]

I added a auto default.

//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?