feat: support useI18n({ messages }) in locale message extraction

[jariz]

In short, adds supports for useI18n({ messages }).
Read more in the full issue: #694

What's supported

• Inline object literals: useI18n({ messages: { en: { ... }, ja: { ... } } })
• Local variable references: const msgs = { ... }; useI18n({ messages: msgs })
• Default imports: import messages from './messages.json'; useI18n({ messages })
• Named imports: import { messages } from './i18n.js'; useI18n({ messages })
• Aliased named imports: import { messages as msgs } from './i18n.js'; useI18n({ messages: msgs })
• TypeScript as expressions: useI18n({ messages: { ... } as const })
• Multiple useI18n() calls in a single file (each produces its own locale message set)
• Combined sources: <i18n> blocks + useI18n({ messages }) + localeDir all merge together
• Nested keys: t('form.submit') resolves correctly against nested message objects
• String, number, boolean, null literal values and static template literals (no expressions)

What's NOT supported (silently skipped)

• Spread elements: useI18n({ messages: { ...base, en: { ... } } }) — the entire messages object is skipped if any spread is encountered since it's not statically analyzable
• Computed property keys: useI18n({ messages: { [locale]: { ... } } }) — skipped
• Dynamic values: useI18n({ messages: getMessages() }) or any non-identifier, non-object expression — skipped
• Namespace imports: import * as msgs from '...' — not resolved
• Non-top-level variable declarations: only top-level const/let/var in the module scope are collected (sufficient for <script setup>)
• Re-exports / indirect references: const a = b; useI18n({ messages: a }) where b is another variable — only one level of indirection is followed
• Template literals with expressions: `${dynamicLocale}` as values — skipped
• Non-existent import paths: if the resolved file doesn't exist on disk, silently skipped

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Latest commit: 3c08384

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