Revise the loglevel API to be more golang idiomatic

[gammazero]

• LogLevel has a String method to convert the numeric level into its string version
  • This also allows the compiler to automatically call the String method when formatting the value (fmt.Print).
• There is only one name used to identify the default log level, and it is declared as a const DefaultName
  • Eliminates confusion over whether to use "", "*", "default", etc.
  • The value of the DefaultName const can change with requiring any change is code that uses the const.
• Removed the "Get" name for functions that lookup a value.
  • "it's neither idiomatic nor necessary to put Get into the getter's name" see Effective Go
• A DefaultLevel function returns the current log level and handles locking.
  • Code does need to worry about whether locking is needed to get the default, let the function do whatever is needed.
• Function to convert string to LogLevel is named Parse
  • This is typical among other golang APIs when converting a string to another type.

[SgtPooki]

Makes sense. we’ll need to change kubo to address the new default name because i think there are expectations there of “*”.

Default name being an empty string or “*” make the same sense to me

[SgtPooki]

Nice

[rvagg]

This is really hurting downstream compatibility leading to the usual wac-a-mole of getting dependencies to work together (this being one of the dependencies that's used almost everywhere in our little ecosystem). Since it was a breaking change this would have been a good reason to /v3. In lieu of that, can we have these back please as aliases marked as deprecated?