Add XCConfig parsing support

[EvanBacon]

Summary

• Adds a new xcconfig module for parsing and manipulating Xcode configuration files (.xcconfig)
• Implements single-pass parser with support for #include/#include? directives, conditional settings, and variable expansion
• Integrates with XCBuildConfiguration to read and flatten base configuration files

Features

• Parser: Single-pass parser for xcconfig format
• Include resolution: #include and #include? directive support with file path resolution
• Conditional settings: Support for [sdk=*], [arch=*], [config=*] conditions
• Variable expansion: $(VAR) references and $(inherited) keyword
• Build settings flattening: Merge includes with optional platform filtering
• Writer: Serialize parsed configs back to xcconfig format

Test plan

• Unit tests for parser with various xcconfig formats
• Tests for include directive resolution
• Tests for conditional settings filtering
• Tests for writer round-trip

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