Fix unicode arrow icon not rendering with some fonts in configuration editor

[garrettjstevens]

The "level arrow" in the ConfigurationEditor is currently "🡒" (Rightwards Sans-Serif Arrow U+1F852), which does not render on my computer, so it looks like this:

Since I recently re-installed my OS, I'm guessing there are other systems that have trouble with that arrow character as well. This PR changes it to "➔" (Heavy Wide-Headed Rightwards Arrow U+2794), which I think is more common. It looks like this for me:

If someone with a Mac (@rbuels or @carolinebridge-oicr?) could double-check for me that it looks fine on there, that would be great. Session link: https://s3.amazonaws.com/jbrowse.org/code/jb2/config_editor_level_arrow/index.html?session=share-uNPMnDdnXA&password=QunGJ

[carolinebridge]

looks great!

[codecov]

Codecov Report

Merging #3142 (f4e963e) into main (ad9bf14) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3142      +/-   ##
==========================================
- Coverage   58.81%   58.79%   -0.03%     
==========================================
  Files         648      648              
  Lines       29042    29042              
  Branches     7067     7067              
==========================================
- Hits        17082    17075       -7     
- Misses      11581    11588       +7     
  Partials      379      379              

Impacted Files	Coverage Δ
...ionEditorWidget/components/ConfigurationEditor.tsx	89.18% <ø> (ø)
...svg/src/SvgFeatureRenderer/components/Segments.tsx	85.71% <0.00%> (-7.15%)	⬇️
...src/SvgFeatureRenderer/components/FeatureLabel.tsx	80.55% <0.00%> (-5.56%)	⬇️
...gins/svg/src/SvgFeatureRenderer/components/util.ts	91.83% <0.00%> (-4.09%)	⬇️
...FeatureRenderer/components/ProcessedTranscript.tsx	87.32% <0.00%> (-1.41%)	⬇️
packages/core/util/layouts/GranularRectLayout.ts	86.86% <0.00%> (-0.43%)	⬇️
products/jbrowse-web/src/util.ts	48.33% <0.00%> (ø)

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