Migrate from tslint to @rubensworks/eslint-config

[Copilot]

Replaces the deprecated tslint setup with ESLint using the @rubensworks/eslint-config flat config.

Config changes

• Remove tslint, tslint-eslint-rules; add @rubensworks/eslint-config, eslint@^8
• eslint.config.js — flat config with file ignores and type-aware parsing
• tsconfig.eslint.json — extends base tsconfig to include test/** for type-aware linting
• Updated lint script: eslint . --cache
• Added .eslintcache to .gitignore

Code fixes (lib/TermUtil.ts)

• eslint-disable/enable blocks around <any> casts in termToString (existing TS bug workaround — microsoft/TypeScript#26933)
• Wrapped case blocks containing declarations in {} (no-case-declarations)
• Added u flag to all regex literals (require-unicode-regexp)
• || → ?? for dataFactory default assignment (ts/prefer-nullish-coalescing)
• Renamed type params Q → TQ (ts/naming-convention)
• Reformatted long lines; removed stale // Tslint: comments

Test fixes (test/TermUtil-test.ts)

• Removed return from all return expect(...) patterns (jest/no-test-return-statement)
• Added error message strings to bare .toThrow() calls (jest/require-to-throw-message)
• Shortened an over-length test description; renamed a duplicate it title

[coveralls]

coverage: 100.0%. remained the same
when pulling 559fa12 on copilot/migrate-from-tslint
into b23b491 on master.