Document missing elements in theme()

[arcresu]

This closes #6827 by adding documentation for the missing theme elements (mostly palettes), along with a new unit test to prevent regression.

Happy to change the wording of course. There were many choices to group the palette params and I'm not sure of the best place to point to explain the values accepted.

[arcresu]

Ah, the test failure is coming from a different place and is now exposed by the new params for theme():

 ══ Failed tests ════════════════════════════════════════════════════════════════
 ── Failure ('test-theme-sub.R:48:3'): theme elements are covered in `theme_sub_*()` functions ──
 Snapshot of code has changed:
 old[12:16] vs new[12:24]
     [17] "axis.minor.ticks.r"            "axis.ticks.length.theta"      
     [19] "axis.ticks.length.r"           "axis.minor.ticks.length.theta"
     [21] "axis.minor.ticks.length.r"     "axis.line.theta"              
 -   [23] "axis.line.r"                   "complete"                     
 -   [25] "validate"                     
 +   [23] "axis.line.r"                   "palette.colour.discrete"      
 +   [25] "palette.colour.continuous"     "palette.fill.discrete"        
 +   [27] "palette.fill.continuous"       "palette.alpha.discrete"       
 +   [29] "palette.alpha.continuous"      "palette.linewidth.discrete"   
 +   [31] "palette.linewidth.continuous"  "palette.size.discrete"        
 +   [33] "palette.size.continuous"       "palette.shape.discrete"       
 +   [35] "palette.shape.continuous"      "palette.linetype.discrete"    
 +   [37] "palette.linetype.continuous"   "strip.placement.x"            
 +   [39] "strip.placement.y"             "complete"                     
 +   [41] "validate"     

So I guess the question is do we also need theme_sub_palette()? I'll add for now.

[arcresu]

The remaining CI failure appears to be unrelated to the changes.