feat: implemented langchain adapter and added TrimConfig

[IBJunior]

• Implemented compressLangChainHistory
• Moved messagesToKeep in TrimConfig interface

[Copilot]

Pull Request Overview

This PR implements a LangChain adapter for the slimcontext library, adding convenient integration with LangChain's message format and chat models. The main purpose is to provide a seamless way to compress LangChain message histories using slimcontext's compression strategies without requiring manual conversion between message formats.

Key changes include:

• Added a comprehensive LangChain adapter with message conversion utilities and a high-level compression function
• Introduced a TrimConfig interface to formalize the configuration structure for trim compression
• Added optional peer dependency support for @langchain/core with proper package exports

Reviewed Changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/adapters/langchain.ts	New adapter module with message conversion utilities and compressLangChainHistory function
tests/langchain.test.ts	Comprehensive test suite for the LangChain adapter functionality
src/strategies/trim.ts	Extracted TrimConfig interface and updated constructor to use it
src/index.ts	Added export for the langchain adapter module
package.json	Added LangChain peer dependency, package exports, and version bump
examples/LANGCHAIN_COMPRESS_HISTORY.md	New example demonstrating one-call history compression
README.md	Updated documentation with LangChain adapter section
CHANGELOG.md	Added release notes for version 2.1.0
.github/copilot-instructions.md	Added comprehensive repository documentation
.eslintignore	Added locales directory to ignore list

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported