Some fixes

[pepicrft]

Description 📝

• Change PBXBuildFile.file attribute to be of type PBXFileElement.
• Add PBXBuildPhase.add(file:) method that takes a file element and returns a build file.
• Add PBXProj.rootObject attribute.
• Fix XCBuildConfiguration.baseConfiguration type.

[ghost]

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Generated by 🚫 dangerJS

[codecov]

Codecov Report

Merging #297 into master will decrease coverage by 0.25%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
- Coverage   74.13%   73.87%   -0.26%     
==========================================
  Files         116      116              
  Lines        5447     5466      +19     
==========================================
  Hits         4038     4038              
- Misses       1409     1428      +19

Impacted Files	Coverage Δ
...es/xcodeproj/Objects/BuildPhase/PBXBuildFile.swift	80.64% <ø> (ø)	⬆️
...j/Objects/Configuration/XCBuildConfiguration.swift	62.85% <ø> (ø)	⬆️
Sources/xcodeproj/Objects/Project/PBXProj.swift	72.22% <0%> (-18.48%)	⬇️
...s/xcodeproj/Objects/BuildPhase/PBXBuildPhase.swift	34.4% <0%> (-3.24%)	⬇️

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