Add tightness property to List node

[johnxnguyen]

What's in this PR?

This PR adds the isTight property to the List node. List tightness is a hint to a renderer whether it should apply extra spacing between list items or not.

Closes #214

Note

Usage of this property to adjust the list item spacing will be addressed in a separate PR.

[iwasrobbed-ks]

Looks 👍 just needs a test update before merging please 🙏

[johnxnguyen]

@rob-keepsafe yep, just having issues building the test target on my machine, will try to fix it as soon as I have some free time.

[codecov]

Codecov Report

Merging #215 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #215   +/-   ##
=======================================
  Coverage   68.63%   68.63%           
=======================================
  Files          74       74           
  Lines        2047     2047           
=======================================
  Hits         1405     1405           
  Misses        642      642           

Impacted Files	Coverage Δ
Source/AST/Nodes/List.swift	88.23% <100.00%> (ø)

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