feat: add message replacement for log records

Add Logger.AddReplacer to mask or normalize message text before records reach writers.
Document replacement behavior in README and package GoDoc, and update the example.

Also fix JSON writer edge cases:
- avoid panic on empty message lists
- preserve newline semantics when the last message already ends with n
- keep original message argument types for custom writers when no replacers are configured

Update dependencies and add release notes for the next release.

go.mod

@@ -2,10 +2,10 @@ module git.nix13.pw/scuroneko/slog
 
 go 1.26
 
-require github.com/fatih/color v1.18.0
+require github.com/fatih/color v1.19.0
 
 require (
 	github.com/mattn/go-colorable v0.1.14 // indirect
-	github.com/mattn/go-isatty v0.0.20 // indirect
-	golang.org/x/sys v0.42.0 // indirect
+	github.com/mattn/go-isatty v0.0.21 // indirect
+	golang.org/x/sys v0.43.0 // indirect
 )