Generate BuildableCollectionNodes with SwiftSyntaxBuilder

- Stub out BuildableCollectionNodes generation
- Generate elements initializer in BuildableCollectionNodes
- Generate combining initializer
- Generate arrayLiteral initializer
- Generate build method implementation
- Add buildSyntax implementation
- Add expressibleAs conformance method
- Add createSyntaxBuildable
- Generate Array conformances
- Bootstrap BuildableCollectionNodes
- Use FunctionSignatures in InitializerDecls
- Use new IfStmt convenience init in BuildableCollectionNodesFile
- Use new builder-based ExprList initializers
- Use new builder-based ConditionElementList initializer
- Use new VariableDecl convenience initializer
- Use new SequenceExpr initializer
- Factor out utility method for format-leadingTrivia params
- Factor out generated functions in BuildableCollectionNodesFile
- Parenthesize leading trivia correctly

Author: fwcd

Package.swift

@@ -77,7 +77,6 @@ let package = Package(
       dependencies: ["SwiftSyntax"],
       exclude: [
         "gyb_helpers",
-        "BuildableCollectionNodes.swift.gyb",
         "BuildableNodes.swift.gyb",
         "ResultBuilders.swift.gyb",
       ]