chore: add counts keyword to stats/base/dists/poisson/mgf

Add the `counts` keyword to `package.json` to align with the
within-namespace majority pattern: 12 of the 14 sibling packages in
`stats/base/dists/poisson` carry `counts` (85.7% conformance), with
`ctor` the other outlier. The MGF of a Poisson distribution
characterizes the moment-generating function of a count-valued random
variable, so the omission is unintentional metadata drift.

Inserted between `discrete` and `poisson` to match the sibling
convention used by `pmf`, `logpmf`, and `quantile`.

Metadata only; no source, behavior, test, or documentation changes.

Author: claude

lib/node_modules/@stdlib/stats/base/dists/poisson/mgf/package.json

@@ -62,6 +62,7 @@
     "generating functions",
     "moments",
     "discrete",
+    "counts",
     "poisson",
     "univariate"
   ]