Fix issue where snackbar would not show up when same error message is triggered multiple times

[cmdcolin]

I found that if you trigger the same snackbar error e.g. two times, then close the snackbar, then trigger it again a third time, the snackbar will not show up.

Found debugging #3388

This fixes the issue by always showing the last message on the snackbar stack. It avoids tricky React state+mobx-state-tree state conflicts by removing all react hooks from method

[codecov]

Codecov Report

Merging #3389 (e7b525f) into main (0dfd9bf) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3389      +/-   ##
==========================================
- Coverage   59.32%   59.31%   -0.01%     
==========================================
  Files         812      812              
  Lines       29761    29744      -17     
  Branches     7160     7155       -5     
==========================================
- Hits        17656    17643      -13     
+ Misses      11917    11913       -4     
  Partials      188      188              

Impacted Files	Coverage Δ
packages/core/ui/Snackbar.tsx	68.42% <100.00%> (-3.81%)	⬇️
products/jbrowse-web/src/util.ts	48.33% <0.00%> (ø)

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[cmdcolin]

there might be a small UI difference compared with main after this change where it closes and re-pops up 100ms later on main, but should be a minor change in exchange for some correctness here