chore(toolkit-lib): rename StackSelector option to expand

[mrgrain]

In #105 I renamed the interface from Extend to Expand, but forgot the options itself and some docs.

This is marked as a chore so it doesn't show up in the changelog for the first release. That's what we want, because it is not relevant to users just yet.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.82%. Comparing base (dee84c1) to head (f83f9bf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   84.44%   84.82%   +0.37%     
==========================================
  Files         196      196              
  Lines       35089    35089              
  Branches     4516     4551      +35     
==========================================
+ Hits        29632    29763     +131     
+ Misses       5308     5184     -124     
+ Partials      149      142       -7     

Flag	Coverage Δ
suite.unit	84.82% <ø> (+0.37%)	⬆️

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