[Chore] Add SwiftLint

[johnxnguyen]

What's in this PR?

• Add SwiftLint configuration and GitHub action.
• Fix violated lint rules

[codecov]

Codecov Report

Merging #252 (5087876) into master (f83e9a2) will decrease coverage by 0.02%.
The diff coverage is 93.54%.

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
- Coverage   91.32%   91.29%   -0.03%     
==========================================
  Files          60       60              
  Lines        1037     1057      +20     
==========================================
+ Hits          947      965      +18     
- Misses         90       92       +2     

Impacted Files	Coverage Δ
Sources/Down/AST/Nodes/BaseNode.swift	100.00% <ø> (ø)
Sources/Down/AST/Nodes/BlockQuote.swift	100.00% <ø> (ø)
Sources/Down/AST/Nodes/Code.swift	100.00% <ø> (ø)
Sources/Down/AST/Nodes/CodeBlock.swift	100.00% <ø> (ø)
Sources/Down/AST/Nodes/CustomBlock.swift	0.00% <ø> (ø)
Sources/Down/AST/Nodes/CustomInline.swift	0.00% <ø> (ø)
Sources/Down/AST/Nodes/Document.swift	100.00% <ø> (ø)
Sources/Down/AST/Nodes/Emphasis.swift	100.00% <ø> (ø)
Sources/Down/AST/Nodes/Heading.swift	100.00% <ø> (ø)
Sources/Down/AST/Nodes/HtmlBlock.swift	100.00% <ø> (ø)
... and 50 more

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