Add unit axis attribute, for Plots v1

[Ickaser]

Description

Essentially identical to #5095 , but made against the master branch so that I'm not waiting for Plots v2--folder structures are different enough that it made sense to open as a separate PR.

@gustaphe and I discussed some in #5095; it would be helpful to get feedback from @t-bltg or @BeastyBlacksmith or someone else who is not as focused on Unitful usage.

A lot of the noise in the diff is a result of switching to Plots.get_guide(axis) instead of axis[:guide], because that new function allows us to more elegantly handle differing unit formats in the guide strings; in any case where the axis has no units, the result of get_guide(axis) is axis[:guide] so not a big change in behavior.

Other changes involve moving some of the machinery out of the UnitfulExt so that it is accessible within Plots for axis handling.

Attribution

• I am listed in .zenodo.json (see How to get acknowledged #3503)

Things to consider

• Does it work on log scales?
• Does it work in layouts?
• Does it work in recipes?
• Does it work with multiple series in one call?
• PR includes or updates tests?
• PR includes or updates documentation?

[t-bltg]

CI is failing on this one.

[codecov]

Codecov Report

Attention: Patch coverage is 98.14815% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.80%. Comparing base (fa65e7d) to head (2e0f267).
Report is 45 commits behind head on master.

Files with missing lines	Patch %	Lines
src/axes.jl	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5098      +/-   ##
==========================================
+ Coverage   89.77%   89.80%   +0.03%     
==========================================
  Files          40       40              
  Lines        8780     8867      +87     
==========================================
+ Hits         7882     7963      +81     
- Misses        898      904       +6     

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[Ickaser]

This PR isn't quite ready yet, because I think it deserves a few more tests for the UnitfulExt before merging, plus some additions to the docs. But @t-bltg the CI failures seem to be either 1) a resurface of a PlotlyJS>Blink>Electron issue #5075, which is randomly hitting only some of the Ubuntu test cases or 2) something to do with the 1.12 prerelease

[t-bltg]

1. a resurface of a PlotlyJS>Blink>Electron issue fix PlotlyJS ECONNREFUSED #5075

#5100

2. something to do with the 1.12 prerelease

Ignore those (they are development aids), we focus on julia release and lts.