Add `medium` size to Pill #3488

Job	Run time
Check for .changeset entries for all changed files (ubuntu-latest, 20.x)	52s
Prime node_modules cache for primary configuration (ubuntu-latest, 20.x)	19s
Publish npm snapshot (ubuntu-latest, 20.x)	1m 59s
Lint (ubuntu-latest, 20.x)	2m 1s
Check build sizes (ubuntu-latest, 20.x)	1m 24s
Test (ubuntu-latest, 20.x, 1/2)	1m 1s
Test (ubuntu-latest, 20.x, 2/2)	1m 5s
	8m 41s

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