DEV: Introduce a helper for handling events (#25433)

Instead of

```hbs
{{on "input" (action this.foo value="target.value")}}
{{on "input" (action (mut this.bar) value="target.value")}}
```

you can use:

```hbs
{{on "input" (with-event-value this.foo)}}
{{on "input" (with-event-value (fn (mut this.bar)))}}
```

or in gjs:

```gjs
import { fn } from "@ember/helper";
import { on } from "@ember/modifier";
import withEventValue from "discourse/helpers/with-event-value";
…
{{on "input" (withEventValue (fn (mut this.bar)))}}
```

plugins/styleguide/assets/javascripts/discourse/components/sections/molecules/char-counter.hbs

@@ -1,10 +1,7 @@
 <StyleguideExample @title="<CharCounter>">
   <CharCounter @max="50" @value={{@dummy.charCounterContent}}>
     <textarea
-      {{on
-        "input"
-        (action (mut @dummy.charCounterContent) value="target.value")
-      }}
+      {{on "input" (with-event-value (fn (mut @dummy.charCounterContent)))}}
       class="styleguide--char-counter"
     ></textarea>
   </CharCounter>