Strip alpha from colors for improved SVG export compatibility

[cmdcolin]

This PR adds a @jbrowse/coreu/util called stripAlpha primarily used for exporting svg colors. The core SVG spec doesn't like the fill to be an rgba, and instead likes fill being rgb and a separate prop for "opacity". Our usage of the theme variables from MUI includes an alpha by default, so this strips it (and does not supply the separate opacity prop). We could make it supply the separate opacity prop automatically e.g. for user defined colors but it's probably better at least for the default theme variables to strip it off. There is an interesting built-in way this could be done with MUI with 'css variables' but it is an experimental API (see channel tokens here https://mui.com/material-ui/experimental-api/css-theme-variables/customization/#channel-tokens)

[cmdcolin]

fixes #3864

[codecov]

Codecov Report

Merging #3865 (f302478) into main (f6a4244) will decrease coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3865      +/-   ##
==========================================
- Coverage   64.46%   64.45%   -0.01%     
==========================================
  Files        1003     1003              
  Lines       29782    29785       +3     
  Branches     7146     7146              
==========================================
- Hits        19198    19197       -1     
- Misses      10421    10425       +4     
  Partials      163      163              

Files Changed	Coverage Δ
packages/core/util/index.ts	80.65% <100.00%> (-0.09%)	⬇️
...ns/svg/src/SvgFeatureRenderer/components/Arrow.tsx	100.00% <100.00%> (ø)
...src/SvgFeatureRenderer/components/FeatureLabel.tsx	80.95% <100.00%> (ø)
...svg/src/SvgFeatureRenderer/components/Segments.tsx	93.75% <100.00%> (ø)

... and 1 file with indirect coverage changes

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