feat: add Nexus Mods username module

[kristoisberg]

This PR adds an username module for the gaming site nexusmods.com. One of the simpler modules yet, since the site exposes E2E testing attributes for nearly everything of interest.

[brunolm]

1. generic_validate (httpx) is Cloudflare-blocked — every request returns 403 (Just a moment...), so the module always errors and never returns taken/available. Live check: same URL via impersonate_request(..., allow_redirects=True) returns 200 for real profiles (Gopher, Pickysaurus, Dark0ne) and 404 for fakes. Switch to impersonate_validate and use allow_redirects=True (curl_cffi kwarg, not follow_redirects). Homepage warmup_url is optional; fingerprint alone was enough here.
2. joined / last_active scrape display text (07 Sept 2004) while the same <time> elements expose ISO dateTime (e.g. 2004-09-07T10:55:16.000Z). Prefer the dateTime attribute for locale-independent, machine-sortable values.
3. Also extract data-e2eid="unique-download-count" (present on live profiles, e.g. 9.4m) into extra, same pattern as the other e2e fields.
4. Avatar URLs are https://avatars.nexusmods.com/<user_id>/100 — optionally emit the numeric id into extra (and/or prefer og:image for the avatar).

Raw output:

 Checking username: brunolm
  [!] Nexusmods [https://www.nexusmods.com/profile/brunolm] (brunolm): Error (Unexpected status code: 403)


[i] Scan complete.
  Total hits: 0

Matches the PR review: httpx hits Cloudflare (403), so the module never gets a profile response. No JSON file was written (nothing to save with only an error and no -o).

[kristoisberg]

@brunolm Thank you for the suggestions, I applied them all. I also parsed some additional information from the page's hydration data.

[kaifcodec]

@kristoisberg This is really a high quality module as it reveals the targets' email.
@brunolm Thanks for review on this.

MERGING IT!