std.log: make current level be a function

[lun-4]

From #2586 (comment)

Making it configurable at runtime would not be something the default thing would do but you could override the log function to achieve that behavior.

And from #2586 (comment)

You could also make default_level a function, which just returns a comptime value by default, or root can make it a function that returns a comptime or runtime value.

Overriding the log function isn't enough because level checking happens before that, so it'd make sense to expose that bit of logic for API users to change.

Overriding the current way level is set also isn't enough because that's set as a const. That leaves an overridable function to be done, as Marler suggested.

Here's an example of how this patch looks in practice:

var current_log_level: std.log.Level = .info; // default

pub fn log_level() std.log.Level {
    return current_log_level;
}

pub fn main() !void {
   // ...
   if (args.options.verbose) current_log_level = .debug;
   // ...
}