Add ability to add custom log level and custom log level label

[chadglaser4682]

I read the doc, but may have missed it. Feel free to point it out if this is already available.

My team is implementing go-hclog in a service, while other services of our services are using slog.
In our slog implementation, we added a level for auditing, in between warn and error. We had the ability to change the name of the level so it outputs the word "audid" where it would normally say "debug" or "info" etc.

We added the audit ability to the go-hclog implementation at log level 6, but I can't seem to get the logger to output the name "audit" I can get the logger to output [?????] or if I turn on json, it outouts @Level:all

slog's level.go explains that it leaves gaps in the level integers, (-4, 0, 4, 8) and we added a log level between 4 and 8. Then we change the name in a function in our own code. But in go-hclog, I can't seem to change the level label, it seems to be added in intlogger.go -> writer.go.

tldr: Custom log level and custom log level labels.