Underscore-prefixed models referenced in public APIs are not exported

[JialinHuang803]

The modular emitter filters out underscore-prefixed models from exports (see buildRootIndex.ts) although they are referenced in the public APIs.

For example:

model _123Test {  // not exported
  id: string;
  name: string;
}

op createTestData(
  testData: _123Test
): _123Test;

[JialinHuang803]

The @typescript-eslint/naming-convention eslint rule is turned off for the generated models.ts files (#3584). Therefore, if the customer defines the model with an identifier starting with '_' followed by a number, it won't be normalized by the emitter or detected by eslint. However, the api-view will still display a warning about this.