Expose `simplify` flags

Hello @donmccurdy

I would like to start supporting the [latest meshoptimizer 0.25](https://github.com/zeux/meshoptimizer/releases/tag/v0.25) version while still using the gltf-transform API to handle everything around it.

For that I believe the `simplifyPrimitive` function needs to expose the [flags array](https://github.com/zeux/meshoptimizer/blob/d24a3e5e1e012233e0953b033cd9c2c9a84436ef/demo/simplify.html#L483):

Start at the simplifyPrimitive API and compare its current options with the meshoptimizer 0.25 simplify flags shown in the linked demo. Confirm how the flags should be exposed through the TypeScript API, then verify that callers can pass them and that the existing simplification behavior remains covered by the relevant tests.

Written by the indexing model from the issue text.