feat(plpgsql-deparser): add context-based RETURN statement handling #915

Job	Run time
parser-tests (parser)	58s
parser-tests (pgsql-cli)	56s
parser-tests (plpgsql-parser)	1m 2s
parser-tests (transform)	5m 51s
parser-tests (plpgsql-deparser)	1m 3s
parser-tests (proto-parser)	1m 1s
parser-tests (deparser)	57s
parser-tests (traverse)	1m 0s
parser-tests (utils)	1m 3s
	13m 51s

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