Add export of `Callback` type

[stefanprobst]

Initial checklist

• [x] I read the support docs
• [x] I read the contributing guide
• [x] I agree to follow the code of conduct
• [x] I searched issues and couldn’t find anything (or linked relevant results below)
• [x] If applicable, I’ve added docs and tests

Description of changes

Adds the Callback type to the root exports, like in the previous unified-engine version.

[codecov-commenter]

Codecov Report

Merging #53 (fa6a1c0) into main (eac1eea) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #53   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         2484      2485    +1     
=========================================
+ Hits          2484      2485    +1     

Impacted Files	Coverage Δ
index.js	100.00% <100.00%> (ø)

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[github-actions[bot]]

Hi! This was closed. Team: If this was merged, please describe when this is likely to be released. Otherwise, please add one of the no/* labels.

[wooorm]

Released! https://github.com/unifiedjs/unified-engine/releases/tag/9.0.1