chore: drop unused @babel/runtime dep and redundant bundled ESM output

- Remove @babel/runtime from dependencies and @babel/plugin-transform-runtime
  from devDependencies. No build output referenced them (grep across cjs, esm,
  umd returned 0 matches), so they were dead weight in the dependency tree.
- Switch rollup's babelHelpers from 'runtime' to 'bundled'. If any helper is
  ever emitted in the future it will be inlined rather than imported from an
  external runtime package.
- Stop emitting dist/esm/i18next.bundled.js. It was byte-identical to
  dist/esm/i18next.js because no helpers were being generated.

Also rebuild umd outputs — root i18next.js / i18next.min.js pick up the
26.0.7 missingKey log change.

Closes #2424