Ability to discern between types directive and x-TypeScript-types

For `deno vendor`, it might be nice to have a way to represent `/// ` and `x-TypeScript-types` separately in the graph while still providing a common method for the `maybe_types_dependency`.

This would also help us remove `Range` from being necessary for `x-typescript-types` as the types directive could have a range, but the `x-typescript-types` doesn't need one:

https://github.com/denoland/deno_graph/blob/f8adbe45ab39a52ec2bbcb671a65593acf072226/src/graph.rs#L1331-L1340

Alternatively, I can just do a check for the start and end position all being zero, but that seems a little hacky.