Miscellaneous improvements to guides

[teunbrand]

This PR aims to fix a series of backward compatibility issues that the GuideOld class didn't (perfectly) solve, in addition to a few polishes. It originates from a reverse dependency check which revealed what issues packages have with the guide system. This exercise has surfaced some problems that are being addressed in this PR. The main issues I have found are:

• The S3 guide extensions that relied upon S3 NextMethod() class inheritance. Since we've gutted these in ggplot2, we had anticipated that these needed to be reimplemented in the package extensions themselves. This is mostly handled in PRs towards these packages.
• We may have been too strict in letting guides() throw an error with empty or unnamed input. Some of the packages relied on guides() == NULL, which is now the case, whereas others provided unnamed guides, which is now a warning instead of an error.
• Every so often a unit test assumed that g <- guides(colour = "colourbar") would store the information in g$colour, whereas this information is now stored in g$guides$colour and variations of this. This also has been addressed in PRs to those packages.

All in all, surveying this has led to the following PRs that are ready for review:

• guide_rampbar() compatibility with ggplot2 >3.4.2 mjskay/ggdist#186
• Guide compatability with ggplot2 >3.4.2 csdaw/ggprism#25
• Use new location of guides yutannihilation/gghighlight#192
• Compatibility with future release ggplot2 eliocamp/ggnewscale#57
• Update tests ashenoy-cmbi/grafify#6
• Update test openanalytics/inTextSummaryTable#3

And PRs that rely on fixes in this PR to become ready for review:

• guide_level() compatibility with ggplot2 >3.4.2 mitchelloharawild/distributional#92
• Guide compatability with ggplot2 >=3.5.0 Sayani07/gghdr#25
• guide compatability with ggplot2 >3.4.2 stefanedwards/lemon#30
• Guide methods eliocamp/metR#177
• WIP: Guide compatability with ggplot2 >3.4.2 davidgohel/ggiraph#270

Unrelated to this PR: besides issues with the guide system, it was also common to see the expect_equal(length(plot), 9) in tests, which should now be 10, or the direct borrowing of internals that we've superseded.

[thomasp85]

LGTM