Add copy token that creates non-existing parent directories

Previously, premake allowed to do something like this on Windows:

postbuildcommands {
  "{COPY} %{wks.location}premake5.lua %{wks.location}this/does/not/exist/"
}

...and it created the parent folders like mkdir -p does on Linux.

We relied on that "feature", but since premake automatically omits trailing slashes now, it results in an error. This is probably intended and always behaved like that on Linux, but I think it's actually a pretty handy feature and might make sense to be officially re-introduced on all platforms.

If adding it to {{COPY}} breaks something, I suggest introducing {{COPYP}} or similar instead.

Start by tracing the implementation of the {COPY} command used by postbuildcommands, especially its handling of trailing slashes on Windows and Linux. Compare the existing platform behavior and check whether the proposed {COPY} change or a separate {COPYP} command fits better. Done means the documented command consistently creates missing parent directories across the supported platforms, with regressions covered by the relevant tests.