Do not rely on short (8+3) names on Windows

[plicease]

Fixes #13, fixes #28.

Short (8+3) names are not created by default on newer versions of Windows, so neither the module nor its tests should depend on them.

Library

• win32_space_be_gone returns the original path unchanged when no short name without spaces is available. Previously it returned whatever Win32::GetShortPathName gave back (the long path, or undef for a path that doesn't exist).
• Docs say the result may still contain spaces, and point to the array-ref form of set_alias.

Tests

• t/lib/TestLib.pm no longer shortens the temp dir. It quotes paths in the generated scripts (cmd, PowerShell, Unix) and in the commands that run them.
• The PowerShell check uses corpus/true.ps1 off MSWin32, so /bin/sh doesn't eat the backslash (the ttrue.ps1 / corpustrue.ps1 errors in Possible quoting issue with power shell on windows breaks test #13 and test failures under WSL #31).
• alias_array.t no longer shortens the perl path; the array form quotes each word.
• alias.t (string form, where quoting is up to the caller) skips when the perl or script path contains a space; alias_array.t covers that case.
• space_win32.t checks for no spaces when short names exist, and for unchanged paths when they don't.

Testing

• Linux: passes normally, and also with a space in both the temp dir and the perl path. On main, that setup fails 8 test files.
• Not tested on Windows: the cmd.exe and PowerShell branches and space_win32.t need a Windows run.

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