feat(theme): add queryString option to localeDropdown

[wceolin]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

This PR adds a queryString option to the localeDropdown considering @slorber's review for #5890

Test Plan

@slorber mentioned we should dogfood it but I wasn't sure how do it. I can think of two ways for dogfooding it:

1. Full integration test that sets the queryString option and uses Netlify Edge Functions to set a cookie to remember a user's preference.
2. A simpler test to check if the queryString is properly appended to the URL.

I manually tested the following cases:

• Should add a query string when the config is passed.
• Should not add a query string when the config is undefined.
• Should not keep the query string when navigating to another route.

I'm happy to write tests for those cases (and additional ones I didn't think of) but I'll need some pointers on how to get started.

Test links

Deploy preview: https://deploy-preview-8915--docusaurus-2.netlify.app/

Related issues/PRs

#5890
#5839 (reply in thread)

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 75	🟢 97	🟢 92	🟢 100	🟢 90	Report
/docs/installation	🟠 82	🟢 100	🟢 92	🟢 100	🟢 90	Report

[slorber]

Thanks 👍

That looks good enough for me and is not too risky to merge.

We'll look into using this on our own website later and when Netlify edge functions are out of beta