tsp format should clean up unused imports

[baywet]

Clear and concise description of the problem

If I have the following two typspec files

main.tsp

import "./models.tsp";

model Foo {
  bar: String
}

models.tsp

model Bun {
  prop: String
}

I'd expect the tsp format command to clean up the unused import in the main file. Currently it does not.

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