added isassigned method for StringArray to avoid disastrous try/catch fallback

[ExpandingMan]

The AbstractArray fallback for Base.isassigned relies on a try/catch and is unbelievably slow.

This PR adds a method for StringArray to avoid this along with some tests.

[codecov]

Codecov Report

Merging #95 (4723c23) into main (40c0d5e) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   76.39%   76.47%   +0.07%     
==========================================
  Files           2        2              
  Lines         627      629       +2     
==========================================
+ Hits          479      481       +2     
  Misses        148      148              

Impacted Files	Coverage Δ
src/WeakRefStrings.jl	78.26% <100.00%> (+0.14%)	⬆️

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[quinnj]

Thank you!