Make bossbar use `EntityLayer` for visibility #1433

Job	Run time
valence-fmt (ubuntu-latest, default, nightly-2023-06-20)	36s
valence-docs (windows-latest, default, stable)	7m 27s
valence-docs (macos-latest, default, stable)	3m 48s
valence-docs (ubuntu-latest, default, stable)	2m 31s
extractor-tests (windows-latest)	2m 50s
extractor-tests (macos-latest)	2m 57s
extractor-tests (ubuntu-latest)	2m 8s
valence-clippy (windows-latest, default, stable)	9m 10s
valence-clippy (macos-latest, default, stable)	6m 56s
valence-clippy (ubuntu-latest, default, stable)	3m 21s
valence-tests (windows-latest, default, stable)	15m 24s
valence-tests (macos-latest, default, stable)	13m 7s
valence-tests (ubuntu-latest, default, stable)	8m 33s
	1h 18m 48s

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