Add Always Use Inline Functions With Assign as an xstate til

xstate/always-use-inline-functions-with-assign.md

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+# Always Use Inline Functions With Assign
+
+The [`assign`
+action](https://xstate.js.org/docs/guides/context.html#assign-action) allows
+you to update the context of the state machine.  There are a couple ways to use
+`assign`, but you should prefer the inline function style.
+
+Technically `assign` can be passed an object literal like so to set a static
+value:
+
+```javascript
+{
+  actions: {
+    resetCount: assign({
+      count: 0
+    })
+  }
+}
+```
+
+However, this causes confusion for the TypeScript compiler and can create
+surprising and unrelated type errors in the parts the code that use the
+machine.
+
+To keep the compiler happy, you should instead use inline functions. Either
+like this:
+
+```javascript
+{
+  actions: {
+    resetCount: assign((_context) => {
+      return {
+        count: 0
+      }
+    })
+  }
+}
+```
+
+or like this:
+
+```javascript
+{
+  actions: {
+    resetCount: assign({
+      count: (_context) => 0
+    })
+  }
+}
+```