`:s[ubstitute]` not handling backslashes correctly

Describe the bug
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I have a line of text like C:\\Program Files\\Microsoft and I want to replace all double backslash (\\) to single backslash (\), so I tried below command line substitution:

:s/\\\\/\\/g
or
:s/\\\\/\/g
or
:s/(\\)\\/$1/g

The first command did nothing, and so I tried the second one, which replaced double backslash (\\) to single slash (/). Only the third one worked as what I expected.

Both 1st and 3rd command worked well in gVim 8.2.12 (though need to replace $1 to \1).

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
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Screenshots
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If remapping-related, please attach log output: https://github.com/VSCodeVim/Vim#debugging-remappings.

Environment (please complete the following information):

• Extension (VsCodeVim) version:1.20.2
• VSCode version:1.56.0
• OS: Windows 10

Additional context
Add any other context about the problem here.

Start at the :s[ubstitute] command entry point and reproduce the three substitutions described in the issue using VSCodeVim on Windows. Trace how backslashes and replacement references are parsed, then verify that replacing double backslashes with single backslashes behaves as expected without changing them to slashes.