Non-English search support

#1081 has been stuck for a while, so I tried implementing my own version.

Preview the search functionality online

Inspired by #1496.

Major implementation steps:

• Land #2392
  • Fix buggy teaser (preview) ^1
    • Keyword highlighting doesn't work as expected for Chinese
    • The selected range displayed as teaser sometimes contains no keywords at all (the target chapter does contain the keyword though)
  • Enhance mixed multi language search support
    • Implement a "fallback" strategy for text in mixed languages (detail)
• Support Emoji search
  • Fix elasticlunr-rs's indexing implemantation (detail)
• Localize UI rendered by HTML renderer

Unresolved questions:

• Should search-non-english feature be enabled by default? (Not likely, since it causes severe binary size bloat)
  • Should each supported language have an individual feature flag? Would it make the feature flag list too noisy?
• Should we consider other search solutions than elasticlunr.js & lunr-languages?
• Should the new "fallback" search implementation apply to English books as well?

Start by reviewing #2392 and the linked implementation details, then examine the elasticlunr-rs indexing work and the HTML renderer localization scope. Done means the remaining checklist items are resolved and the feature-flag, search-solution, and fallback-strategy questions have decisions.