fix(nuxt): resolve vue3 runtime plugin on Nuxt 4

[CodeDredd]

The module resolved its runtime plugin via isNuxt3() ? '3' : '2', so any Nuxt major other than 3 — including Nuxt 4 — fell back to the Nuxt 2 plugin and the app broke.

This inverts the check to isNuxt2(nuxt) ? '2' : '3': only Nuxt 2 gets the vue2 plugin, every newer major (3, 4, …) resolves the vue3 plugin.

isNuxtMajorVersion() is not available in the @nuxt/kit version pinned on 1.x (3.12.3, added in 3.13), so the still-supported isNuxt2() is used instead. On main (v2) the module no longer has the vue2/vue3 split, so this only affects the 1.x line.

Verified with a local nuxt-module-build build.

fixes #2019

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.44%. Comparing base (edc781f) to head (1c6878d).
⚠️ Report is 1 commits behind head on 1.x.

Additional details and impacted files

@@           Coverage Diff           @@
##              1.x    #2026   +/-   ##
=======================================
  Coverage   99.44%   99.44%           
=======================================
  Files          86       86           
  Lines        3048     3048           
  Branches      531      531           
=======================================
  Hits         3031     3031           
  Misses         14       14           
  Partials        3        3           

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