Mark units defined in this package as `public`

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).

Start by locating the @unit macro and the unit definitions in this package, then inspect how bindings are currently exported or marked public. The change is done when package-defined units, including SI-prefixed units, can be looked up without the internal-binding warning; no specific test file is named in the issue.

Written by the indexing model from the issue text.