Replace tslint with ESLint and lint in CI

[isc-klu]

npm run lint has been unusable: tslint 5 (deprecated since 2019) crashes on TypeScript 5 (no-shadowed-variable "threw an error" ×20), and tslint:recommended demands space indentation while .editorconfig and every file use tabs, so it reported 2679 indent errors plus 132 others. It is not run in CI, so nothing caught this.

Changes

• tslint → eslint + typescript-eslint, both recommended presets, in a flat eslint.config.mjs. The stale .eslintrc.json/.eslintignore copied from vscode-objectscript years ago (and never wired up) are removed with tslint.json.
• Rules relaxed to match the codebase's existing conventions: no-explicit-any off, unused function args and catch bindings allowed (tsconfig already has noUnusedParameters: false), ignoreRestSiblings for the { accessToken: _, ...rest } idiom, empty catch {} allowed. Everything else is default.
• The 21 findings fixed with no behavior change: Function → a GetChildren signature, namespace StorageIds → a const object, {} header type → Record<string, string>, hasOwnProperty via a hasServer helper, new Array() → [], one let → const, one non-null assertion on an optional chain, and a targeted disable for the lazy require("node-cmd"). 13 dead // tslint:disable-next-line comments removed.
• npm run lint added to main.yml and prerelease.yml before compile.
• skipLibCheck in tsconfig.json: ESLint 10's bundled types conflict with @types/eslint-scope pulled in by webpack. (Integration tests against IRIS containers for release-preparation PRs #359 needs the same flag for @vscode/test-electron 3, so the two PRs will touch the same line.)

Not changed

Formatting is left to .editorconfig; no Prettier, so no reformatting diff. tsfmt.json is untouched.

🤖 Generated with Claude Code