Bug 22059 – std.file: Long paths support on Windows requires explicit \\?\ prefix

``` void main() { import std.file; // works assert(exists(`\\?\C:\Users\Martin\AppData\Local\dub\packages\mir-algorithm-1.0.1\mir-algorithm\.dub\build\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D.obj`)); // fails assert(exists(`C:\Users\Martin\AppData\Local\dub\packages\mir-algorithm-1.0.1\mir-algorithm\.dub\build\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D.obj`)); } ``` The prefix shouldn't be required and added automatically, as the Visual Studio 2019 linker doesn't require that prefix, nor does the LLD linker, nor does DMD, ... Phobos should essentially use something very similar to DMD's extendedPathThen() helper: https://github.com/dlang/dmd/blob/ef39b070819216a8d9835e2a66d3e365420763ec/src/dmd/root/filename.d#L1146

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