lsp-rust: report command capabilities #5332

Job	Run time
test (ubuntu-latest, 27.2, false)	2m 31s
test (ubuntu-latest, 28.2, false)	2m 32s
test (ubuntu-latest, 29.4, false)	2m 54s
test (macos-latest, 28.2, false)	3m 29s
test (macos-latest, 29.4, false)	3m 53s
test (windows-latest, 27.2, false)	3m 30s
test (windows-latest, 28.2, false)	4m 8s
test (windows-latest, 29.4, false)	3m 45s
test (ubuntu-latest, snapshot, true)	1m 20s
test (macos-latest, snapshot, true)	3m 4s
test (windows-latest, snapshot, true)	57s
	32m 3s

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