fix(babel-preset-app): use loose mode for babel class plugin

[clarkdo]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Fix nuxt-start issue from @aldarund , as class plugin is the only builtin plugin of @nuxt/babel-preset-app, so use loose mode for avoiding unnecessary babel runtime helper.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #6435 into dev will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##             dev   #6435   +/-   ##
=====================================
  Coverage   95.6%   95.6%           
=====================================
  Files         79      79           
  Lines       2684    2684           
  Branches     692     692           
=====================================
  Hits        2566    2566           
  Misses       101     101           
  Partials      17      17

Flag	Coverage Δ
#e2e	100% <ø> (ø)	⬆️
#fixtures	50.7% <ø> (ø)	⬆️
#unit	92.28% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
packages/babel-preset-app/src/index.js	96.66% <ø> (ø)	⬆️

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