feat(plugin-orama): add Orama search plugin

[Mister-Hope]

Description

Add a new official search plugin @vuepress/plugin-orama, powered by Orama, mirroring the architecture of @vuepress/plugin-slimsearch.

It provides the same client-side search experience (search modal, suggestions, query/result history, custom fields, full-text indexing) backed by the Orama full-text engine.

Features

• Full-text and CJK search via a custom Intl.Segmenter-based tokenizer, consistent between index building and search
• Per-locale index with worker-based search in both dev and build mode
• Custom fields, indexContent, preserveTags, filter, sortStrategy options (parity with slimsearch)
• Configurable tokenizer per locale via indexOptions / indexLocaleOptions
• Search suggestions (implemented on top of Orama's prefix matching)
• Query / result history, hotkeys, and keyboard navigation
• Docs (EN + ZH) and unit tests

Notes

• indexOptions / indexLocaleOptions accept a tokenizer factory instead of slimsearch's tokenize / processTerm, since Orama uses a schema-based tokenizer component.
• The id field is excluded from searchable properties to avoid spurious matches; threshold: 0 (AND semantics) matches slimsearch's default.