os: implement os.type() using uv_os_uname()
The happy path behavior should be identical on all platforms except MinGW, which now identifies MinGW separately from Windows.
Labeling this as semver-major due to the change in behavior on MinGW. I could also #ifdef
the previous behavior on MinGW, and remove it in a later semver-major commit.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines