Add a move_legend convenience function

[mwaskom]

This addresses issues discussed in #2280, along with some of the issues in #2231

It is a somewhat hack-ish solution. Because matplotlib legends don't offer public
control over their location, this copies data from an existing legend to a new
object, and then removes the original legend. I am hopeful that there will be
upstream changes that make legend repositioning more natural, but this is
a reasonable stopgap measure to alleviate a common seaborn pain-point.

It is also in a sense more powerful than the name suggests, because it lets one
update other legend parameters, which is also not otherwise possible with
public methods on an existing legend object.

Example in action:

ax = sns.histplot(penguins, x="bill_length_mm", hue="species")
sns.move_legend(ax, "lower center", bbox_to_anchor=(.5, 1), ncol=3)

[codecov]

Codecov Report

Merging #2643 (57a08c9) into master (091f4c0) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 57a08c9 differs from pull request most recent head 85e6947. Consider uploading reports for the commit 85e6947 to get more accurate results

@@            Coverage Diff             @@
##           master    #2643      +/-   ##
==========================================
+ Coverage   97.51%   97.53%   +0.01%     
==========================================
  Files          17       17              
  Lines        6402     6439      +37     
==========================================
+ Hits         6243     6280      +37     
  Misses        159      159              

Impacted Files	Coverage Δ
seaborn/utils.py	95.20% <100.00%> (+0.59%)	⬆️

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