Inject more CallerOffset in pterm.NewSlogHandler

[MarvinJWendt]

As discussed in #599

The slog handler for pterm.Logger adds additional function calls, which break the default offset of pterm.Logger. We could inject more offset into the passed pterm.Logger when pterm.NewSlogHandler is called. To make this change non-breaking, we should only inject the offset, if the passed pterm.Logger has no manually configured offset.

/cc @ccremer

[MarvinJWendt]

Injecting the offset is not as easy as thought initially, since the pterm.Logger is passed as a pointer. So changing the offset in pterm.NewSlogHandler would change the offset in the logger itself, which could be used somewhere else. Passing the logger via a pointer has the advantage, that the logger configuration can be changed on the fly, e.g.:

logger := pterm.DefaultLogger.WithCaller()
logger.Info("default logger with caller")

slogger := slog.New(pterm.NewSlogHandler(logger))

slogger.Info("slog logger with caller")
logger.Info("default logger with caller 2")

slogger.Debug("This message should not print")

logger.Level = pterm.LogLevelDebug // <-- This should still be possible

slogger.Debug("This message should print")

So we need to temporarily fork the logger in the slog handler, when it is used.

[MarvinJWendt]

Another benefit of this is, that we can change the offset if the slog package is refactored. Before that change, every repo using PTerm would have to change their manual offset.