feat: Improve plugin load error handling

[mattrunyon]

Loads plugins individually instead of failing to register if any plugin fails to load. Previously if any plugin failed to load for any reason then no plugins were registered.

Does some validation on if the manifest file exists and the structure of it so that the errors are more descriptive. Does not throw a 2nd error about Unexpected token < if there is no manifest file

[codecov]

Codecov Report

Merging #1214 (98e83e1) into main (59d3df4) will increase coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1214      +/-   ##
==========================================
+ Coverage   44.20%   44.25%   +0.04%     
==========================================
  Files         448      450       +2     
  Lines       33437    33488      +51     
  Branches     8404     8427      +23     
==========================================
+ Hits        14781    14820      +39     
- Misses      18606    18618      +12     
  Partials       50       50              

Flag	Coverage Δ
unit	44.25% <0.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/code-studio/src/main/AppInit.tsx	0.00% <0.00%> (ø)
packages/code-studio/src/plugins/PluginUtils.tsx	3.84% <0.00%> (+0.62%)	⬆️

... and 9 files with indirect coverage changes

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[mofojed]

Would recommend throwing an error instead of a string