chore: add guidance on getters/setters to coding standards (#7977)

Author: jelbourn

CODING_STANDARDS.md

@@ -154,6 +154,24 @@ be part of the user-facing API. This typically applies to symbols used in templa
 Additionally, the `@docs-private` JsDoc annotation can be used to hide any symbol from the public
 API docs.
 
+
+#### Getters and Setters
+* Avoid long or complex getters and setters. If the logic of an accessor would take more than
+three lines, introduce a new method to contain the logic.
+* A getter should immediately precede its corresponding setter.
+* Decorators such as `@Input` should be applied to the getter and not the setter.
+* Always use a `readonly` property instead of a getter (with no setter) when possible.
+  ```ts
+  /** YES */
+  readonly active: boolean;
+
+  /** NO */
+  get active(): boolean {
+    // Using a getter solely to make the property read-only.
+    return this._active;
+  }
+  ```
+
 #### JsDoc comments
 
 All public APIs must have user-facing comments. These are extracted and shown in the documentation