Preserve the leading slashes of Windows UNC paths

[Eljees]

Problem

A plain Windows UNC path loses its leading double slash:

normalize('\\\\server\\share\\file.css');
// actual:   '/server/share/file.css'
// expected: '//server/share/file.css'

The leading \\ is treated as a repeated separator and collapsed, so the
result no longer refers to a network path. The Win32 namespace prefixes are
already special-cased (\\?\C:\... -> //?/C:/..., \\.\CdRomX ->
//./CdRomX); a plain UNC path is the one Windows form that loses its
doubled leading slash.

Change

After the existing namespace-prefix handling, keep the // prefix when the
input starts with exactly two backslashes followed by a path character. Two
notes on scope:

• Forward-slash inputs (//foo/bar) are intentionally NOT changed and still
  collapse to /foo/bar, exactly as the existing tests specify - a doubled
  leading slash is only unambiguous on backslash-spelled Windows paths.
• Three or more leading backslashes are not a valid UNC form and keep the
  current collapsing behaviour.

Tests

5 new assertions (plain UNC, admin share c$, trailing-slash stripping and
stripTrailing: false). Full suite: 34 passing before, 39 passing after,
0 failing; all pre-existing expectations untouched.

Downstream context

This is the root cause of stylelint/stylelint#3045 (linting a file addressed
via a UNC path silently reports no results): stylelint normalizes the entry
before handing it to fast-glob, and the collapsed /server/share/... no
longer matches the file. A stylelint maintainer asked whether this could be
fixed upstream here rather than worked around there
(stylelint/stylelint#3045 (comment)).

Verified end to end on a real Windows host: with this patch dropped into
stylelint's node_modules, stylelint's UNC reproducer
(\\localhost\<drive>$\...) lints correctly with no changes on the
stylelint side; with the unpatched file it fails.