fix(toolkit-lib): ToolkitError is not exported

[mrgrain]

Fixes '@aws-cdk/toolkit-lib' has no exported member named 'ToolkitError'

Because we are now moving parts of the public API into a shared package, we also need to export the respective types.
During the regular build, type declarations are not bundled. So we need to take care of it afterwards.
This is not ideal, but required. Thankfully the temporary package is a, well, temporary construct and we are actively working to get rid of it (first by adding new stuff, then by moving everything to toolkit-lib).

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.03%. Comparing base (3710b78) to head (dc15ead).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
- Coverage   85.25%   85.03%   -0.23%     
==========================================
  Files         207      207              
  Lines       35723    35723              
  Branches     4644     4635       -9     
==========================================
- Hits        30457    30377      -80     
- Misses       5114     5185      +71     
- Partials      152      161       +9     

Flag	Coverage Δ
suite.unit	85.03% <ø> (-0.23%)	⬇️

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