mysql.index is not work on property

Nobody has claimed this yet.

## Steps to reproduce

1. Create the model, repository, and detasource with `@property({type: 'string', mysql: {index: {kind: "FULLTEXT"}}})` on model.
2. `npm run migrate` on cli.
3. It should make FULLTEXT INDEX, however it isn't made on MySQL.

This is example of a model.

```ts
// Copyright IBM Corp. and LoopBack contributors 2018,2020. All Rights Reserved.
// Node module: @loopback/example-todo
// This file is licensed under the MIT License.
// License text available at https://opensource.org/licenses/MIT

import {Entity, model, property} from '@loopback/repository';

@model()
export class Todo extends Entity {
@property({
type: 'number',
id: true,
generated: false,
})
id?: number;

@property({
type: 'string',
required: true,
// Not work for index creation
mysql: {
index: {
kind: 'FULLTEXT',
},
},
})
title: string;

@property({
type: 'string',
// work for index creation. but it is diffrent by README
index: {
kind: 'FULLTEXT',
// also it can `unique: true` in here, not mysql.index.unique...
},
})
desc?: string;

constructor(data?: Partial) {
super(data);
}
}

export interface TodoRelations {
// describe navigational properties here
}

export type TodoWithRelations = Todo & TodoRelations;
```

## Current Behavior

`@property.mysql.index` is not work.

But, I see work with `@property.index`.

## Expected Behavior

We can replace `@property.mysql.index` with `@property.index` on README.

https://github.com/loopbackio/loopback-connector-mysql/blob/f8f40a1199b6ed63ba01d6d173a9314004c08c93/README.md?plain=1#L535-L545

Or,

Fix the `prop.index` to `prop.mysql.index` and `m.properties[propName].index;` to `m.properties[propName].mysql.index;` on migration.js.

https://github.com/loopbackio/loopback-connector-mysql/blob/f8f40a1199b6ed63ba01d6d173a9314004c08c93/lib/migration.js#L661

https://github.com/loopbackio/loopback-connector-mysql/blob/f8f40a1199b6ed63ba01d6d173a9314004c08c93/lib/migration.js#L361

## Link to reproduction sandbox

https://github.com/forno/loopback-next/blob/index-bug-report/examples/todo/src/models/todo.model.ts

## Additional information

the index creation function are already exist. But it is different config written by README.
I don't know that either is correct.

## Related Issues

https://github.com/loopbackio/loopback-connector-mysql/issues/350

_See [Reporting Issues](http://loopback.io/doc/en/contrib/Reporting-issues.html) for more tips on writing good issues_

Use examples/todo/src/models/todo.model.ts as the reproduction, run npm run migrate, and inspect the cited locations around lines 361 and 661 in lib/migration.js. Compare those property lookups with the README guidance and the working @property.index example; done means the documented configuration and migration behavior consistently create the intended FULLTEXT index.

Written by the indexing model from the issue text.