fix(config): resolve build options with fallback

[bluwy]

Description

Use mergeConfig when resolving build options, but only for non-array options as we don't want to merge arrays. Array-able options like target and cssTarget fallback in resolved object instead.

Additional context

Fix failing ecosystem CI for Svelte

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
astro	✅ success
histoire	✅ success
iles	✅ success
ladle	✅ success
laravel	✅ success
marko	✅ success
nuxt-framework	✅ success
rakkas	✅ success
svelte	✅ success
vite-plugin-ssr	✅ success
vite-setup-catalogue	✅ success
vitepress	✅ success
vitest	✅ success
windicss	✅ success