Generate BuildableNodes with SwiftSyntaxBuilderGeneration

- Set up BuildableNodesFile
- Generate BuildableNodes members
- Use ...BaseName where needed in BuildableNodesFile
- Generate default initializer in BuildableNodes fully
- Disable assert stmt generation until #549 is fixed
- Generate convenience initializer parameters
- Generate convenience initializer declaration
- Use ParameterClause builder initializer
- Generate initializer delegations
- Generate build method in BuildableNodes
- Generate build base type method in BuildableNodes
- Generate expressible-as conformance in BuildableNodes
- Generate base type ExpressibleAs conformance
- Generate disambiguating conformances in BuildableNodes
- Generate withTrailingComma function in BuildableNodes
- Escape token names in backticks
- Bootstrap BuildableNodes
- Replace deprecated SyntaxFactory in BuildableNodesFile

Author: fwcd

Package.swift

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