Add type aliases (RadioButton and Checkbox) for convenient.

[yoichiro]

Summary

The BlockElementAction already has two types ( RadioButtonsAction and CheckboxesAction ) which were added recently. However, we currently need to specify them as a parameterized type for BlockAction, because there are no type aliases for them.

We will be able to use them more convenient, if their type aliases are provided.

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 #471 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #471   +/-   ##
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  Coverage   84.23%   84.23%           
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  Files           7        7           
  Lines         552      552           
  Branches      163      163           
=======================================
  Hits          465      465           
  Misses         60       60           
  Partials       27       27           

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[stevengill]

Thanks for sending this in @yoichiro! Merging it now

[yoichiro]

@stevengill Thank you for the reviewing and merging! :)

[seratch]

@yoichiro Thanks for taking the time to make this change! I'm glad to see more contributions from Japan in these days 😄

[yoichiro]

@seratch Thank you! And, it would be great if you confirm it, because I submitted the new issue: slackapi/node-slack-sdk#993 :)