Add a centralized logger

[tombruijn]

The JavaScript integration only uses the console.log/warn/debug/error type functions to log things. It would be useful to route this all through a centralized logged so that we have more control over the log format and when we log certain messages, such as implementing the log_level config option.

To do

• Create centralized logger.
• Update all log statements to use the centralized logger (with the exception of deprecation warnings and other messages that should be visible to the user).
• Add log_level config option. See integrations guide.

Related issues

• Similar issue for the Node.js integration: Add a centralized logger appsignal-nodejs#520