Backward compatibility for trans field deprecation: attempt 1

[teunbrand]

This PR aims to fix some reverse dependency problems.

To briefly describe the problem: people are using foo <- Scale$trans all over their code. We have removed the Scale$trans field in #5566, in favour of the Scale$transformation field. All this code is now broken.

Since trans is not a function, it is not easy to deprecate elegantly. This PR uses some active binding voodoo witchcraft to (1) redirect Scale$trans to Scale$transformation and (2) issue a deprecation message when using Scale$trans. This would allow people's code to continue functioning and, by virtue of the annoying-ness of warning messages, hopefully bully them into using the new field.

I've left this PR as a draft, since active bindings are not my bread and butter and I'd like some scrutinizing eyes on this.

A demonstration:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
options(lifecycle_verbosity = "warning")

scale <- scale_x_continuous(transform = "log10")

# Correct transformation is returned as expected
scale$get_transformation()
#> Transformer: log-10 [1e-100, Inf]

# Transformation field itself is also fine
scale$transformation
#> Transformer: log-10 [1e-100, Inf]

# Trans field throws warning, but returns correct transformer
scale$trans
#> Warning: Scale$trans was deprecated in ggplot2 3.5.0.
#> ℹ Please use Scale$transformation instead.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.
#> Transformer: log-10 [1e-100, Inf]

# Even if the trans field is updated, the update is redirected to the transformation field
# Note that get_transformation() returns the updated transformer
scale$trans <- transform_sqrt()
scale$get_transformation()
#> Transformer: sqrt [0, Inf]

# However, even if the field is updated, it continues to throw warnings
scale$trans
#> Warning: Scale$trans was deprecated in ggplot2 3.5.0.
#> ℹ Please use Scale$transformation instead.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.
#> Transformer: sqrt [0, Inf]

^{Created on 2024-01-02 with reprex v2.0.2}

[teunbrand]

Closing this in favour of #5626