Remove some deprecated items

[selfcontained]

Summary

This removes 2 deprecated items:

• Top level FunctionHandler export that was replaced with SlackFunctionHandler type.
• CustomType definitions support for callback_id that was replaced with name.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #91 (bb947af) into main (6650c12) will increase coverage by 1.97%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   96.34%   98.31%   +1.97%     
==========================================
  Files          44       44              
  Lines        1669     1664       -5     
  Branches       90       90              
==========================================
+ Hits         1608     1636      +28     
+ Misses         59       26      -33     
  Partials        2        2              

Impacted Files	Coverage Δ
src/types/mod.ts	100.00% <100.00%> (ø)
src/workflows/mod.ts	94.89% <0.00%> (+7.14%)	⬆️
src/manifest/mod.ts	97.83% <0.00%> (+7.79%)	⬆️
src/workflows/workflow-step.ts	89.10% <0.00%> (+7.92%)	⬆️

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[shapirone]

Let's remove handling from the Custom Type export method as well. Should we do a doc sweep looking for callback_id or FunctionHandler?

[selfcontained]

Let's remove handling from the Custom Type export method as well. Should we do a doc sweep looking for callback_id or FunctionHandler?

Did a pass at the docs and didn't find any mention of them.

[shapirone]

LGTM!