[FR]: Make `ts_config` emit a flat config

### What is the current behavior?

The output of `ts_config()` is the same file as was passed in via the `tsconfig` prop. If said config file makes use of `extends`, the resulting `npm_package` contains an incomplete TS config file with dangling `extends` reference.

### Describe the feature

`tsc` has a `--showConfig` option that can be used to “flatten” the config. It would be nice if the `ts_config` would offer to emit that file instead of the original one (although I am not sure how to deal with the file name conflict).

Start by locating the ts_config rule and how it copies the tsconfig prop into the npm_package output. Compare that behavior with tsc --showConfig, then determine how the flattened output should be named without a file conflict; done means configs using extends produce a self-contained emitted TypeScript config with no dangling extends reference.

Written by the indexing model from the issue text.