Rename heartbeats to cron check-ins

See appsignal/appsignal-elixir#957.

Emit a deprecation warning both when the `heartbeat` helper is used
and when the `Heartbeat` class is initialised, but only once each.

.changesets/deprecate-heartbeats.md

@@ -0,0 +1,6 @@
+---
+bump: patch
+type: deprecate
+---
+
+Calls to `Appsignal.heartbeat` and to the `Appsignal.Heartbeat` constructor will emit a deprecation warning.

.changesets/rename-heartbeats-to-cron-check-ins.md

@@ -0,0 +1,22 @@
+---
+bump: patch
+type: change
+---
+
+Rename heartbeats to cron check-ins. Calls to `Appsignal.heartbeat` and `Appsignal.Heartbeat` should be replaced with calls to `Appsignal.checkIn.cron` and `Appsignal.checkIn.Cron`, for example:
+
+```js
+// Before
+import { heartbeat } from "@appsignal/nodejs"
+
+heartbeat("do_something", () => {
+  do_something()
+})
+
+// After
+import { checkIn } from "@appsignal/nodejs"
+
+checkIn.cron("do_something", () => {
+  do_something
+})
+```