Accept 'native' time unit where documented as supported

[rickard-green]

Fixes erlang:monotonic_time(native), erlang:system_time(native), erlang:time_offset(native), and os:system_time(native).

Fixes #6165

[github-actions[bot]]

CT Test Results

       3 files     125 suites   39m 43s :stopwatch: 1 455 tests 1 413 :heavy_check_mark: 42 :zzz: 0 :x: 1 754 runs  1 695 :heavy_check_mark: 59 :zzz: 0 :x:

Results for commit 1867cff4.

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