feat: add update-available detection

lib/version-check/index.ts

@@ -0,0 +1,7 @@
+export { startScheduler, stopScheduler, checkOnce } from './sender';
+export { loadState, saveState, effectiveEndpoint, disabledByEnv } from './state';
+export { fetchStatus, parseStatus } from './fetcher';
+export type {
+  UpdateStatus, UpdateSeverity, VersionCheckStateFile,
+} from './types';
+export { DEFAULT_VERSION_ENDPOINT } from './types';