Add a readOnly function to svelte/store

[Crisfole]

This is a single, tree-shakable function added to the svelte/store runtime. I re-write this for almost every project, it's a handy way to create a custom store and prevent directly modifying a store from outside a module. The custom-element tests all failed, and I couldn't work out how to add a test for this since the ideal test would be something like:

import {writable, readOnly} from 'svelte/store';
function test() {
  const writableStore = writable(null);
  const readableStore = readOnly(writableStore);

  assertNotNull(writableStore.set);
  assertFalsey(readableStore.set);
}

I'm happy to make any changes needed, or for this to be rejected, but the easiest way to get the conversation going is with a PR.

[Crisfole]

Someone suggested using derived with the id function. That's a similar result, just more complex...

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[bluwy]

Looks like #6518 duplicates this and already had two approvals there. Code changes are similar too.