Use more accurate estimator for feature label widths

[cmdcolin]

The code for estimating feature label widths is a heuristic to avoid having to measure them precisely with getBBox or similar. This updates the heuristic to use a slightly better measurement from @jbrowse/core called measureText, and adds a small "padding" too

[codecov]

Codecov Report

Merging #2106 (a2e1af5) into main (591ec2c) will increase coverage by 0.01%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main    #2106      +/-   ##
==========================================
+ Coverage   61.98%   62.00%   +0.01%     
==========================================
  Files         476      476              
  Lines       22790    22791       +1     
  Branches     5349     5350       +1     
==========================================
+ Hits        14126    14131       +5     
+ Misses       8382     8378       -4     
  Partials      282      282              

Impacted Files	Coverage Δ
...src/SvgFeatureRenderer/components/FeatureLabel.tsx	75.00% <75.00%> (-3.58%)	⬇️
packages/core/util/index.ts	79.89% <100.00%> (+1.32%)	⬆️
...gFeatureRenderer/components/SvgFeatureRendering.js	55.64% <100.00%> (-0.36%)	⬇️

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