Add FAQ entry about client-side init functions

[rmunn]

Demonstrate how to guarantee that a client-side initialization function will be called only once per client.

After writing this comment explaining how to call a library's init function only once on the client, I thought that that might be a generally-useful technique that should be documented. The FAQ section on integrations seems like the right place to put it.

No tests or changesets because this is a documentation-only PR. If a changeset should be added even to doc-only PRs, let me know and I'll be happy to add one.

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[rmunn]

The unit test failure is likely a flaky test that needs fixing, as it failed due to a timeout rather than an actual error. Plus, since this PR made no code changes, it's almost certainly not caused by the PR. I don't seem to have sufficient repo permissions to request a re-run of all jobs on the failing workflow, though. Someone with write access to the repo will probably have to do that.

[Rich-Harris]

If no unsubscription is required, isn't this equivalent?

// clientConnection.ts
import { initalizeConnection } from 'some-library';
export const connection = initializeConnection('params');

import { connection } from '$lib/clientConnection.ts';
function getResult(query) {
	return connection.get(query);
}

Added a similar comment to #1538

[paolodina]

initalizeConnection typo

[paolodina]

initalization typo

[Rich-Harris]

Going to close this as I don't think there's any advantage to the store-based approach — thanks