Add @public

[moble]

Julia 1.11's public keyword functions much the same as export, so I think it makes sense to have the corresponding @public macro in this package.

Here's the summary of things I've done:

1. Moved the processing out of @export into new method handle(::Expr,::Symbol).
2. Created @public
3. Created the no-op version: NoExport.@public
4. Moved include("NoExport.jl") to the top of the main file, which enables...
5. Used the no-op version of @public on older Julia versions.
6. Tested on fixed handle(::Val{:macrocall}, expr) to deal with expressions inside blocks with docstrings
7. Described @public in the README.
8. Added lots of tests, including tests that use Julia>=1.11 capabilities.
9. Ran the test suite (all tests passed) on Julia versions I happened to have installed:
   • 1.0 (which required removing the Manifest.toml)
   • 1.7
   • 1.8
   • 1.9
   • 1.10
   • 1.11
   • 1.12.0-beta3

Note that item 5 has the nice feature of letting us use @public, even for code that will run on the old versions of Julia.

[PatrickHaecker]

LGTM