feat: add ignoreDiacritics option for accent-insensitive matching

[tkirda]

Closes #893.

What

New opt-in option ignoreDiacritics (default false). When true, the default lookupFilter and formatResult ignore accents, so typing cafe finds Café in a local lookup and the highlight wraps the original Café.

• Folding is done one character at a time (NFD → strip combining marks → NFC → lowercase), with a map from folded text back to the source string. That keeps highlights on the original characters, including accents stored as separate combining marks.
• Only diacritics are folded; ß is not treated as ss.
• For Ajax lookups the server still decides what matches; the option only affects highlighting there.

API

formatResult and lookupFilter now receive the resolved options as an extra, optional last argument, so the defaults can read the flag. Existing custom callbacks and direct calls to the defaults are unaffected. The non-folding highlight path produces the same output as before.

Docs

README: new option row, updated callback signatures, and a Styling example for highlighting with <mark> instead of <strong> by wrapping the default formatter (the other half of #893).

Tests

9 new specs: off by default, filtering, highlighting, decomposed marks, repeated matches, ß, HTML escaping, a query that folds to nothing, and an end-to-end lookup.

🤖 Generated with Claude Code

[tkirda]

Closing in favour of README recipes (see #893): accent-insensitive filtering is a one-line custom lookupFilter, and a built-in option plus the callback signature change isn't worth the permanent API surface for one request. Branch feat/ignore-diacritics is kept in case highlighting of accent-folded matches is requested again.