[XcodeGen] Handle 'rule' declarations and generate command line args

* Rename 'BuildRule to 'BuildEdige' because it is the official term
* NinjaParser to handle 'include' and 'rule' directives
* NinjaParser to handle parse "rule name" in 'build' correctly
* Make variable table a simple `[String: String]` and keep any bindings
  to make the substitutions possible.
* Generate command line argumets using 'command' variable in the 'rule'
  and use it as the source of truth, istead of using random known
  bindings like 'FLAGS'.

utils/swift-xcodegen/Sources/SwiftXcodeGen/Path/AnyPath.swift

@@ -52,6 +52,15 @@ extension AnyPath {
       a
     }
   }
+
+  public func absolute(in base: AbsolutePath) -> AbsolutePath {
+    switch self {
+    case .relative(let r):
+      r.absolute(in: base)
+    case .absolute(let a):
+      a
+    }
+  }
 }
 
 extension AnyPath: Decodable {