Add `Slack::BlockKit::Element::NumberInput`

[CGA1123]

Closes: #169

[codecov[bot]]

Codecov Report

Merging #174 (72c66b9) into master (a6ea275) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
+ Coverage   99.15%   99.16%   +0.01%     
==========================================
  Files          45       46       +1     
  Lines         942      962      +20     
==========================================
+ Hits          934      954      +20     
  Misses          8        8              

Files Changed	Coverage Δ
lib/slack/block_kit/element/number_input.rb	100.00% <100.00%> (ø)

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