Add lemma `pmf_measurable` #4947

Job	Run time
rocq-elpi	1m 0s
rocq-core	1m 3s
coq	59s
stdlib	1m 4s
coq-elpi	1m 4s
hierarchy-builder	1m 1s
mathcomp-finmap	1m 9s
mathcomp-bigenough	1m 0s
mathcomp	1m 11s
mathcomp-analysis-single	18m 10s
mathcomp-classical	1m 8s
mathcomp-reals	59s
mathcomp-experimental-reals	1m 2s
mathcomp-reals-stdlib	1m 3s
mathcomp-analysis	16m 25s
mathcomp-analysis-stdlib	1m 10s
	49m 28s

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