devel/signal-fts5-extension: Add new port

Signal-FTS5-Extension is a C ABI library which exposes a FTS5 tokenizer function
named signal_tokenizer that:

Segments UTF-8 strings into words according to Unicode standard
Normalizes and removes diacritics from words
Converts words to lower case

When used as a custom FTS5 tokenizer this enables application to support CJK
symbols in full-text search.