Support `--context` flag of `dune ocaml-merlin` #4154

Job	Run time
Coverage (4.14.x)	4m 38s
Build and Test (macos-latest, 4.14.x)	7m 44s
Build and Test (ubuntu-latest, 4.14.x)	5m 41s
Build and Test (ubuntu-latest, 5.0.x)	3m 52s
Build and Test (ubuntu-latest, 5.1.x)	3m 56s
Build and Test (windows-latest, 4.14.x)	13m 23s
	39m 14s

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