Multiline support

[ansel1]

Sort multiline attr values to the end, and add newlines to the beginning and end of the value, so the value is printed completely on its own lines, e.g. :

2025-01-09 16:12:48 INF main.go:215 > hello world logger=console-slog error=boom size=5 errorVerbose=
boom

main.testVariousHandlers
        /Users/regan/dev/flume/v2/cmd/demo/main.go:215
main.main
        /Users/regan/dev/flume/v2/cmd/demo/main.go:78
runtime.main
        /opt/homebrew/Cellar/go/1.23.4/libexec/src/runtime/proc.go:272
runtime.goexit
        /opt/homebrew/Cellar/go/1.23.4/libexec/src/runtime/asm_arm64.s:1223
2025-01-09 16:12:48 INF main.go:216 > hello world logger=console-slog error=boom size=5 flavor=vanilla