Conditionalize search behind experimental-search feature flag

[zzorba]

Move search module and SearchIndexStoreKind usage behind #[cfg(feature = "experimental-search")] so builds without the feature don't pull in search dependencies. The transitive dependencies of the experimental-search feature in the matrix-sdk crate are currently breaking 'wasm' target builds.

• I've documented the public API Changes in the appropriate CHANGELOG.md files.
• This PR was made with the help of AI.

Signed-off-by: Daniel Salinas

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.92%. Comparing base (2ec031b) to head (d42bfbe).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6539   +/-   ##
=======================================
  Coverage   89.92%   89.92%           
=======================================
  Files         381      381           
  Lines      105914   105914           
  Branches   105914   105914           
=======================================
+ Hits        95243    95247    +4     
+ Misses       7029     7014   -15     
- Partials     3642     3653   +11     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

---

_{Comparing filament-dm:conditionalize-experimental-search (d42bfbe) with main (2ec031b)}

[poljar]

Doh, very true that this can't work on Wasm.

This keeps on happening, I wonder if CI should attempt to build or at least cargo check the bindings for Wasm support.