Idea: Add new hgetallMap method to return Map<String,String>

[TysonAndre]

(and hgetallMapBuffer: Map<String, Buffer>)

Motivation:

• Make it easier to write consise code that avoids prototype pollution
• Often, I'd end up converting Object.entries(await redis.hgetall(key)) to a Map anyway.
  (easier to make callers write map.has(fieldName) than Object.prototype.hasOwnProperty.call(obj, fieldName), since obj[fieldName] would be wrong for fields that are part of the object prototype)

Related to #1267

(Aside: would key in obj work better than obj[key] for the check if the property was already defined, in the PR fixing that issue for hgetall (to account for Object.prototype.customProperty = false;)? Otherwise, the PR looked correct.)

[luin]

(and hgetallMapBuffer: Map<String, Buffer>)

I definitely think it's useful to add it. However, I'm not sure if this is the right time to add it. For now, although ioredis is 100% written in TypeScript, many types are missing (ex there are no typings for Redis#get). I think we should solve the typing issue first and then add more methods to Redis/Commander since the process of adding types may result in breaking changes.

(Aside: would key in obj work better than obj[key] for the check if the property was already defined...

Good point! I didn't think of that. Can you take a look #1418 ?