Fix/count loop perf

[lupodevelop]

This pull request introduces several improvements, both in functionality and infrastructure, for the str library. The main changes include enhanced Unicode correctness in string reversal, performance optimizations in substring counting, and improvements to the CI workflow for more reliable builds.

Unicode correctness and string utilities

• The reverse/1 function now uses the BEAM standard library's grapheme segmentation (string.to_graphemes) for improved Unicode correctness and consistency throughout the library.

Performance optimizations

• The internal implementation of count/3 was refactored to avoid repeated list.length calls within recursive loops, enhancing performance on long strings by passing the length as an argument.

Continuous Integration improvements

• Added a new GitHub Actions workflow (.github/workflows/ci.yml) that pins the Gleam version and improves cache usage for faster and more reproducible CI runs.

Documentation and metadata updates

• Updated the changelog to reflect the new features, optimizations, and CI improvements in version 1.2.0.
• Bumped the package version to 1.2.0 in gleam.toml.

[Copilot]

Pull request overview

This PR introduces performance optimizations and Unicode correctness improvements to the str library, alongside enhanced CI infrastructure. The main focus is on improving the efficiency of substring counting operations and standardizing grapheme handling throughout the library.

Key Changes

• Refactored count/3 internal implementation to eliminate repeated list.length calls in recursive loops by tracking the haystack length as a parameter
• Migrated reverse/1 to use Gleam's standard string.to_graphemes for consistent Unicode handling across the library
• Added a new GitHub Actions CI workflow with pinned Gleam version and optimized caching strategy

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/str/core.gleam	Optimized count_loop by introducing count_loop_with_len helper that tracks list length; simplified reverse/1 to use stdlib string.to_graphemes instead of custom tokenizer; removed unused str/tokenize import
gleam.toml	Bumped version from 1.1.1 to 1.2.0 for new minor release
CHANGELOG.md	Documented performance improvements, Unicode correctness changes, and CI enhancements for version 1.2.0
.github/workflows/ci.yml	Added CI workflow with Gleam 1.13.0, dependency caching, format checking, and test execution

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