Having a prefix to the logger name

[seratch]

Description

As discussed slackapi/bolt-js#278 (comment) , the logger name can be more specific and clearer.

• Add web-api: prefix to https://github.com/slackapi/node-slack-sdk/blob/%40slack/web-api%405.8.0/packages/web-api/src/logger.ts#L19
• Add rtm-api: prefix to https://github.com/slackapi/node-slack-sdk/blob/%40slack/rtm-api%405.0.3/packages/rtm-api/src/logger.ts#L19

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.