Modernize dev tooling: remove unused deps and migrate Jest to Vitest

[ChanTsune]

Summary

Modernize dev tooling: remove unused dev dependencies, then migrate the test runner from Jest to Vitest (with type-checking of tests preserved).

Notes

Cleanup — dropped standard (declared but never run by any script; lint uses ESLint), nock (not imported by any test), and jest-util (redundant direct dep, already provided transitively). Removed the dead standard config block.

Jest → Vitest — replaced jest + ts-jest with Vitest 4, dropping @types/jest, eslint-plugin-jest, jest, and ts-jest. This also removes their large transitive tree — including @babel/core and js-yaml, the sources of the recently fixed Dependabot alerts. Net dependency change: ~440 → ~215 packages.

• vitest.config.mts uses globals + node environment. The .mts extension loads the config as ESM, avoiding the deprecated Vite CJS config API.
• The two tests are converted from Jest's done-callback style to synchronous tests: Vitest passes a TestContext (not done) as the first argument and has no done.fail. Behavior is preserved.
• ESLint: removed the jest plugin and jest-only globals (jest, fit, x*), added vi.
• tsconfig: types jest → vitest/globals; build scoped to src/ so the new root config file is not pulled into the tsc program.

Test type-checking — Vitest transpiles tests via esbuild and does not type-check them (ts-jest did). To keep that coverage, the test script runs vitest run --typecheck. Added tsconfig.vitest.json (extends the build config, noEmit, rootDir "." so test/ can be included alongside src/), referenced from test.typecheck.tsconfig.

Verification

• npm run all (build → format → lint → package → test) passes: 4 test files / 10 tests, no type errors; npm run format-check clean.
• npm ls @babel/core js-yaml ts-jest jest → all absent from the lock file.
• Verified vitest run --typecheck fails (exit 1) on an injected type error in a test file.
• Local vitest verified via --experimental-require-module (local Node is 22.9.0; CI runs Node 24, which supports require(ESM) natively).