Move to new eng package, split build to CJS+ESM

[bwateratmsft]

Plus some other things:

• Fix a lot of new lint issues
• Turn the testing client extension into pure JS so it doesn't have to be built
• Update some dependencies
• Fix tests that broke after updating dependencies (yaml added new diagnostics and vscode-languageserver-textdocument stopped including newlines at the end of TextDocument.getText())

[Copilot]

Pull request overview

This PR modernizes the build infrastructure by migrating to the @microsoft/vscode-azext-eng package and implementing dual CJS/ESM output. Key changes include upgrading to ES2022, Node 22+, updating the ESLint configuration to the new flat config format, converting the test client extension to pure JavaScript, and fixing various lint issues that surfaced after dependency updates.

Key Changes:

• Migrated from CommonJS to dual CJS+ESM build output with separate dist/cjs and dist/esm directories
• Updated TypeScript target and module settings from ES2018/CommonJS to ES2022/ESM with bundler resolution
• Replaced custom ESLint configuration with @microsoft/vscode-azext-eng package standards
• Updated multiple dependencies including vscode-languageserver (8→9), yaml (2.2.2→2.8.2), and testing libraries

Reviewed changes

Copilot reviewed 43 out of 47 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tsconfig.json	Updated to ES2022 target/lib, es2022 module with bundler resolution, output to dist/esm
package.json	Added dual module exports (CJS/ESM), new build scripts, updated to Node >=22, migrated to @microsoft/vscode-azext-eng
eslint.config.mjs	New flat ESLint config using azExtEslintRecommendedTypeChecked preset
.eslintrc.json (deleted)	Removed legacy ESLint config in favor of new flat config
src/test/**/*.test.ts	Removed unnecessary async keywords, updated import paths, fixed lint issues
src/test/clientExtension/*	Converted TypeScript files to JavaScript, removed tsconfig, updated paths to dist/cjs
src/service/**/*.ts	Fixed lint issues: updated named group access syntax, added eslint-disable comments, improved error handling
.vscode/*	Updated tasks/launch configs for new build structure
bin/docker-compose-langserver	Updated require path from lib/ to dist/cjs/

Files not reviewed (1)

• src/test/clientExtension/package-lock.json: Language not supported

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