Mark units defined in this package as public

[sostock]

Currently, looking up docstrings for units produces a warning, since they are neither exported nor public.

help?> Unitful.W
  │ Warning
  │
  │  The following bindings may be internal; they may change or be
  │  removed in future versions:
  │
  │    •  Unitful.W

  Unitful.W

  The watt, an SI unit of power, defined as 1 J / s.

  Dimension: 𝐋^2 𝐌 𝐓^-3.

  See also: Unitful.J, Unitful.s.

The units defined by this package should be marked as public to avoid this.

We could ad an optional argument to the @unit macro that specifies whether the units should be marked public (so that users don’t have to do it individually for every SI prefix).