Added Delannoy number function

[krish8484]

Delannoy is one of the most common function used in Competitive Programming, therefore typed a dynamically programmed function for the same.

[codecov-io]

Codecov Report

Merging #89 into master will decrease coverage by 0.06%. The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   95.58%   95.51%   -0.07%     
==========================================
  Files           8        8              
  Lines         611      624      +13     
==========================================
+ Hits          584      596      +12     
- Misses         27       28       +1

Impacted Files	Coverage Δ
src/numbers.jl	98.86% <92.3%> (-1.14%)	:arrow_down:

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[krish8484]

Please do suggest any changes required or any extra additions needed to this PR. Thanks.