fix: switch to mutation style for vite config mutations

[DylanPiercey]

Description

The Vite config plugin hook exposes two apis. You can either mutate the provided config, or return a new config that is deep merged.

For some reason we were doing both which may have been leading to some issues.

This PR updates the code in that plugin hook to only use the mutation style api.

Checklist:

• I have updated/added documentation affected by my changes.
• I have added tests to cover my changes.

[changeset-bot]

🦋 Changeset detected

Latest commit: bd9a98f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@marko/vite	Patch

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[codecov]

Codecov Report

Base: 89.47% // Head: 89.54% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (bd9a98f) compared to base (92d047c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   89.47%   89.54%   +0.07%     
==========================================
  Files           6        6              
  Lines         285      287       +2     
  Branches       94       95       +1     
==========================================
+ Hits          255      257       +2     
  Misses         23       23              
  Partials        7        7              

Impacted Files	Coverage Δ
src/index.ts	94.38% <100.00%> (+0.05%)	⬆️

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