Add slog support

[sagikazarmark]

This PR adds log/slog support the following way:

• On Go 1.21 and above you can use log/slog as usual
• On Go 1.20 and below Viper falls back to golang.org/x/exp/slog

Under the hood, Viper uses github.com/sagikazarmark/slog-shim to provide support slog on Go 1.20 and below.

At the same time, this PR breaks backwards compatibility with github.com/spf13/jwalterweatherman. Enabling logging through jww no longer works after this change, but since there are no compile errors the only result is no logs being output.

Enabling logging is possible by using the new WithLogger option:

v := viper.NewWithOptions(viper.WithLogger(slog.Default()))

Further changes:

• Trace level logs are now logged as Debug
• The previously experimental WithLogger option is changed to accept an *slog.Logger and is now publicly available
• Logger interface is deprecated and no longer in use (will be removed in a later release)

Fixes #1270 Fixes #1372

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