fix(nuxt): give plugins `.mjs` extension so they're processed

[danielroe]

https://github.com/nuxt/nuxt/issues/24733 is caused by Nuxt not treating .ejs files as JavaScript (and therefore not running certain build plugins on the code within them).

We might be able to handle this transparently within Nuxt as well but this PR fixes it in a backwards-compatible way.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e0410e5) 74.64% compared to head (3bfb4d3) 74.64%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1479 +/- ## ======================================= Coverage 74.64% 74.64% ======================================= Files 28 28 Lines 3849 3849 Branches 264 264 ======================================= Hits 2873 2873 Misses 976 976 ```

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[posva]

This ends up breaking in the playground as if the initializeAuth() function gets called too much. I will see if I can pinpoint and fix the problem, if not, I will revert this for the time being.

[posva]

It was just missing the .client in the name too 😄 4514650

[danielroe]

oops - sorry! 😬

[posva]

No worries