[Chore] Reorganize project structure for SPM

[johnxnguyen]

What's in this PR?

• Reorganize project structure to follow the SPM convention
• Update to swift tools 5.3
• Specify resources in Package.swift (fixes: [Crash] Missing resource bundle when using SPM #243)

[codecov-io]

Codecov Report

Merging #248 (70f6892) into master (4c1c888) will increase coverage by 5.24%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
+ Coverage   33.17%   38.41%   +5.24%     
==========================================
  Files          75       80       +5     
  Lines        2062     2262     +200     
==========================================
+ Hits          684      869     +185     
- Misses       1378     1393      +15     

Impacted Files	Coverage Δ
Sources/Down/AST/Nodes/BaseNode.swift	100.00% <ø> (ø)
Sources/Down/AST/Nodes/BlockQuote.swift	0.00% <ø> (ø)
Sources/Down/AST/Nodes/ChildSequence.swift	77.77% <ø> (ø)
Sources/Down/AST/Nodes/Code.swift	0.00% <ø> (ø)
Sources/Down/AST/Nodes/CodeBlock.swift	0.00% <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	0.00% <ø> (ø)
Sources/Down/AST/Nodes/Heading.swift	0.00% <ø> (ø)
... and 75 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4c1c888...70f6892. Read the comment docs.