[CLI] Prevent usage of already used keywords

@erogleva がすでに取り組んでいます。

2020年10月9日 から。

## Description / Steps to reproduce / Feature proposal

When using the CLI I can create a model named Entity, or Model. The model is generated but no warning will be emitted (this keyword is already used by Loopback).

## Current Behavior

The model/controller/datasource/... is generated.

```ts
import {Model, model, property} from '@loopback/repository';

@model()
export class Model extends Model {
constructor(data?: Partial) {
super(data);
}
}

```

## Expected Behavior

Multiple options here:

1. add warnings in the creation process
2. prevent the component (model/controller/...) from being created
1. by retrying
2. by exiting
3. modify the generated file , for a model e.g:

```ts
import * as repository from '@loopback/repository';

@repository.model()
export class Model extends repository.Model {
constructor(data?: Partial) {
super(data);
}
}
```

## Keywords to consider

For controllers: nothing (overwriting existing controllers is already handled)

For datasources: nothing (overwriting existing datasources is already handled)

For models:
- Entity
- Model

For repositories:
- Repository

For services: ?

For openapi: nothing

Anything else ?

## Acceptance criteria

Add a Yeoman prompt validator to reject model/repository/controller names that are the same as one of the built-in ones. This way, when a user enters invalid name, Yeoman will print an error and repeat the prompt.

- [ ] Enhance `ArtifactGenerator.promptArtifactName()` to allow subclasses (specific generators) to further customize the validation rules for artifact name. For example, we can introduce a protected property that subclasses can override.

https://github.com/strongloop/loopback-next/blob/752db84f3e6f4984c35c2fc65f54eacae23e002e/packages/cli/lib/artifact-generator.js#L60

For each of the following generators, provide validation rule that will reject problematic names:

- [ ] [`lb4 controller`](https://github.com/strongloop/loopback-next/tree/752db84f3e6f4984c35c2fc65f54eacae23e002e/packages/cli/generators/controller/index.js)
- [ ] [`lb4 datasource`](https://github.com/strongloop/loopback-next/tree/752db84f3e6f4984c35c2fc65f54eacae23e002e/packages/cli/generators/datasource/index.js)
- [ ] [`lb4 interceptor`](https://github.com/strongloop/loopback-next/tree/752db84f3e6f4984c35c2fc65f54eacae23e002e/packages/cli/generators/interceptor/index.js)
- [ ] [`lb4 model`](https://github.com/strongloop/loopback-next/tree/752db84f3e6f4984c35c2fc65f54eacae23e002e/packages/cli/generators/model/index.js)
- [ ] [`lb4 observer`](https://github.com/strongloop/loopback-next/blob/752db84f3e6f4984c35c2fc65f54eacae23e002e/packages/cli/generators/observer/index.js)
- [ ] [`lb4 relation`](https://github.com/strongloop/loopback-next/tree/752db84f3e6f4984c35c2fc65f54eacae23e002e/packages/cli/generators/relation/index.js)
- [ ] [`lb4 repository`](https://github.com/strongloop/loopback-next/tree/752db84f3e6f4984c35c2fc65f54eacae23e002e/packages/cli/generators/repository/index.js)
- [ ] [`lb4 service`](https://github.com/strongloop/loopback-next/tree/752db84f3e6f4984c35c2fc65f54eacae23e002e/packages/cli/generators/service/index.js)

---

## 🎆 Hacktoberfest 2020

Greetings :wave: to all Hacktoberfest 2020 participants!

Here are few tips 👀 to make your start easier, see also #6456:

- Before you start working on this issue, please leave a comment to let others know.
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