Migrate to ESLint 9

[Copilot]

Summary

Migrates the project from ESLint 8 (legacy .eslintrc.* configuration format) to ESLint 9 (flat config format).

Changes

Dependencies (package.json)

• eslint: ^8.52.0 → ^9.39.4
• @typescript-eslint/eslint-plugin + @typescript-eslint/parser replaced by typescript-eslint@^8.57.2 — the official unified package that bundles the parser and plugin, plus provides the config() and configs helpers for flat config
• Added @stylistic/eslint-plugin@^2.13.0 — replaces formatting rules removed in ESLint 9 core (semi, indent, quotes, no-extra-semi) and in @typescript-eslint v8 (@typescript-eslint/semi, @typescript-eslint/indent, @typescript-eslint/member-delimiter-style)
• Added globals@^15.15.0 — provides environment globals for flat config (replaces the old env key)

Configuration

• Deleted all .eslintrc.json / .eslintrc.base.json / .eslintignore files
• Created eslint.config.base.js — a shared factory function using tseslint.config() and tseslint.configs.base for TypeScript package configs
• Created eslint.config.js at the root (for root-level JS files) and in each package directory

Rule changes

• Formatting rules renamed from @typescript-eslint/* / core to @stylistic/* (same behavior, just the canonical ESLint 9 location)
• Removed no-buffer-constructor — removed from ESLint 9 core (the Buffer() constructor has been deprecated in Node.js since v6)
• Removed two spurious eslint-disable-next-line no-console comments in jsonrpc/src/common/connection.ts and jsonrpc/src/common/events.ts — the lines use RAL().console, not console, so the disable comments were never needed; ESLint 9 correctly flags them as unused

Lint scripts

• Removed --ext ts from all lint scripts (the flag was removed in ESLint 9; file extensions are now controlled via files globs in the flat config)

Verification

• ✅ npm run lint (all packages) — no errors; pre-existing indentation warnings unchanged
• ✅ npm run compile — TypeScript build passes
• ✅ npm test — all unit/integration tests pass (177 tests across textDocument, types, jsonrpc, protocol, server packages)

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[rzhao271]

@copilot consider replacing the typescript eslint plugins with typescript-eslint package.

[Copilot]

@copilot consider replacing the typescript eslint plugins with typescript-eslint package.

Done in 60bab62. Replaced @typescript-eslint/eslint-plugin + @typescript-eslint/parser with the unified typescript-eslint package. Updated eslint.config.base.js to use tseslint.config() helper and tseslint.configs.base for the parser/plugin setup.

[rzhao271]

@copilot Double-check whether stylistic is still needed now that there's typescript-eslint.

[Copilot]

@copilot Double-check whether stylistic is still needed now that there's typescript-eslint.

@stylistic/eslint-plugin is still needed. typescript-eslint v8 removed all formatting rules (semi, indent, quotes, member-delimiter-style, no-extra-semi) — they were migrated to @stylistic/eslint-plugin in @typescript-eslint v7 and are no longer part of the plugin at all. Confirmed: none of those five rules exist in the typescript-eslint v8 plugin. The stylistic config exposed by typescript-eslint only covers TypeScript-specific patterns like array type style and type assertion consistency, not text formatting.