docs: Add JS code snippets for forms (#54577)

https://nextjs.org/docs/app/building-your-application/data-fetching/forms-and-mutations

Some of the code blocks where I only specified TS code snippets aren't
working with JS as well (when there are no types). I think I might have
incorrectly assumed you could leave those out. So added the JS snippets
in here (same code, just different file names).

docs/02-app/01-building-your-application/02-data-fetching/03-forms-and-mutations.mdx

@@ -185,6 +185,17 @@ export default async function submit() {
 }
 ```
 
+```js filename="app/actions.js" switcher
+'use server'
+
+import { revalidatePath } from 'next/cache'
+
+export default async function submit() {
+  await submitForm()
+  revalidatePath('/')
+}
+```
+
 Or invalidate a specific data fetch with a cache tag using [`revalidateTag`](/docs/app/api-reference/functions/revalidateTag):
 
 ```ts filename="app/actions.ts" switcher
@@ -198,6 +209,17 @@ export default async function submit() {
 }
 ```
 
+```js filename="app/actions.js" switcher
+'use server'
+
+import { revalidateTag } from 'next/cache'
+
+export default async function submit() {
+  await addPost()
+  revalidateTag('posts')
+}
+```
+
 </AppOnly>
 
 ### Redirecting
@@ -244,6 +266,19 @@ export default async function submit() {
 }
 ```
 
+```js filename="app/actions.js" switcher
+'use server'
+
+import { redirect } from 'next/navigation'
+import { revalidateTag } from 'next/cache'
+
+export default async function submit() {
+  const id = await addPost()
+  revalidateTag('posts') // Update cached posts
+  redirect(`/post/${id}`) // Navigate to new route
+}
+```
+
 </AppOnly>
 
 ### Form Validation
@@ -740,7 +775,18 @@ You can set cookies inside a Server Action using the [`cookies`](/docs/app/api-r
 import { cookies } from 'next/headers'
 
 export async function create() {
-  const cart = await createCart():
+  const cart = await createCart()
+  cookies().set('cartId', cart.id)
+}
+```
+
+```js filename="app/actions.js" switcher
+'use server'
+
+import { cookies } from 'next/headers'
+
+export async function create() {
+  const cart = await createCart()
   cookies().set('cartId', cart.id)
 }
 ```
@@ -789,6 +835,17 @@ export async function create() {
 }
 ```
 
+```js filename="app/actions.js" switcher
+'use server'
+
+import { cookies } from 'next/headers'
+
+export async function create() {
+  const auth = cookies().get('authorization')?.value
+  // ...
+}
+```
+
 </AppOnly>
 
 ### Deleting Cookies
@@ -833,6 +890,17 @@ export async function create() {
 }
 ```
 
+```js filename="app/actions.js" switcher
+'use server'
+
+import { cookies } from 'next/headers'
+
+export async function create() {
+  cookies().delete('name')
+  // ...
+}
+```
+
 See [additional examples](/docs/app/api-reference/functions/cookies#deleting-cookies) for deleting cookies from Server Actions.
 
 </AppOnly>