Migrate to ESLint 9 with flat config

[Copilot]

Summary

Migrates the project from ESLint 8 to ESLint 9 using the new flat config format.

Changes

Dependencies (package.json)

• Replaced eslint@^8.57.0 → eslint@^9.0.0
• Removed @typescript-eslint/eslint-plugin@^7.13.1 and @typescript-eslint/parser@^7.13.1
• Added typescript-eslint@^8.57.0 (new unified package supporting ESLint 8, 9, and 10)
• Added @stylistic/eslint-plugin@^5.10.0 (TypeScript stylistic rules moved here in typescript-eslint v8: semi, indent, member-delimiter-style)
• Added globals@^15.15.0 (for Node.js global declarations in flat config)

ESLint Configuration

• Created root eslint.config.mjs in ESLint 9 flat config format, replacing .eslintrc.base.json
• Created per-package eslint.config.mjs for each subpackage (tsc, tooling, protocol, language-service, lsif, sqlite, npm, tsc-tests) that imports the root config and adds package-specific TypeScript projectService.defaultProject settings
• Removed all legacy .eslintrc.json and .eslintignore files

Notes

• The projectService.defaultProject setting in per-package configs ensures type-aware rules like @typescript-eslint/no-floating-promises work correctly with each package's tsconfig.json
• Pre-existing lint warnings (indentation) remain unchanged — they existed before the migration
• Pre-existing build failures (better-sqlite3 native module and @types/node incompatibilities) are unrelated to this ESLint migration

Security Summary

No security vulnerabilities introduced. All new dependencies were verified against the GitHub Advisory Database with no issues found.

[rzhao271]

@copilot redo to fix merge conflicts and broken CI

[Copilot]

@copilot redo to fix merge conflicts and broken CI

Done in b425a7b. Synced with main (PR #208) by updating: CI YAML files to Node.js 22.22.0, @types/node to 22.19.15, sqlite imports back to vscode-languageserver-types, and protocol/language-service engines to >=22. The ESLint 9 migration changes are preserved on top.

[rzhao271]

@copilot redo dependency installation to resolve merge conflicts

[Copilot]

@copilot redo dependency installation to resolve merge conflicts

Done in 5f7b695. Regenerated package-lock.json using --lockfile-version=2 to match upstream's format, minimizing divergence from main and resolving the merge conflicts.