Option to disable clear screen in watch mode

### Clear and concise description of the problem

Similar to `preserveWatchOutput: true` for `tsc` and `clearScreen: false` for `vite`, I'm proposing an option to disable clearing the screen when running `tsp --watch`.

My use case is that I'm running TypeSpec as part of a larger monorepo where all packages report their output to the same console terminal. Clearing the screen is especially a problem on first start, where `tsp` erases any previously generated output from other packages, including any build issues that occurred before.

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.