Add initial `TransformStream` CSVReader implementation

Add initial `TransformStream` CSVReader implementation

This change enables users to create a `TransformStream` by doing `new CSVReader()` instead of always creating an underlying source `ReadableStream`. This behavior is useful when there is some transformation that must be performed on the source file before parsing (e.g., decompressing a `.csv.gz` file using `DecompressionStream`).