Prepare v1.1.0 with API, q-gram, bug-fix, and threading updates

[matthieugomez]

Summary

This PR prepares the 1.1.0 release.

Changes

• add similarity as the canonical public API and deprecate compare
• bump the package version to 1.1.0
• define deterministic q-gram behavior when both inputs are shorter than q
  identical inputs now return distance 0.0
  different inputs now return distance 1.0
• update README/examples and benchmark script naming to use similarity

Bug fixes

• make findnearest and findall ignore missing entries instead of failing
• add regression tests for missing handling and short-string q-gram edge cases
• fix stale docs/examples for pairwise and nearest-match examples

Performance

• rework the threading model to use coarse-grained Threads.@threads :dynamic loops in pairwise, q-gram preprocessing, findnearest, and findall
• remove the previous many-small-task @spawn pattern
• benchmarked current vs previous HEAD on the same synthetic workload
  multithreaded results improved materially on the main paths, especially pairwise
  single-threaded results were mostly improved as well, with modest regressions on q-gram search

Validation

• Pkg.test()
• julia --compiled-modules=no --threads 4,1 --project=. -e "using StringDistances, Test; include(\"test/runtests.jl\")"
• current-vs-HEAD benchmark comparison using a temporary harness on the same deterministic dataset