Empty out the package, have it load the Unitful extension only

[Ickaser]

This may subject to discussion still. But after commenting out a couple tests for already-deprecated functionality, tests on this package happen to pass, and this provides a way to make sure people who added UnitfulLatexify to their Project.toml won't accidentally load Unitful at the same time as this package.

[Ickaser]

Per the discussion in JuliaPhysics/Unitful.jl#808 (comment).

Docs build fails because Unitful 1.25 isn't actually released yet; there may be other errors as well. It probably makes sense to release this after Unitful gets a release, but maybe the docs shouldn't even be building anyway, I don't know.

[gustaphe]

One way to solve the docs issue is to replace the docs by a deprecation notice. Then it will build, and people who use old versions of packages will know to look at an older version of the docs.

[Ickaser]

I think that this is okay now, although probably #23 should get merged into the gh-pages branch first, since that adds the v1.7 docs pages that this docs deploy will expect to be present. It might make sense to wait for Unitful 1.25 release so that the 2.0 here will be installable but that shouldn't be a huge issue I think.

Once this is merged and released, we should make sure that JuliaRegistries/General#138160 is still a valid PR.

[Ickaser]

In trying to address JuliaPlots/Plots.jl#5174, we need a way to load the extension for Julia <1.9. To try and make that work, this PR now uses a (possibly-fragile) hack to reach into the Unitful files and load the Latexify extension. It seems to work in some manual testing on WSL for my machine, but is a little tricky to test.

[Ickaser]

Since Plots is dropping the Julia 1.6 compat going forward, there's no longer a need to conditionally load the extension here--this is back to just emptying the package.