Check srcpath with lstat in ensureSymlinkSync, like ensureSymlink

[abhu85]

symlinkPathsSync checks srcpath with fs.existsSync, while the async symlinkPaths uses fs.lstat. existsSync follows symlinks, so the two disagree when srcpath is itself a broken symlink:

fs.symlinkSync('/tmp/missing-target', '/tmp/broken-src')

await fse.ensureSymlink('/tmp/broken-src', '/tmp/link-a')  // OK
fse.ensureSymlinkSync('/tmp/broken-src', '/tmp/link-b')    // Error: absolute srcpath does not exist

They also differ on a missing srcpath: ensureSymlink rejects with an ENOENT error (ENOENT: no such file or directory, ensureSymlink '...'), while ensureSymlinkSync throws a plain Error('absolute srcpath does not exist') (or 'relative ...') with no code.

This PR uses lstatSync in both branches of symlinkPathsSync, with the same error rewrite as the async version, so the sync API behaves like the async one.

User-visible changes to the sync API:

• a srcpath that is itself a broken symlink is accepted, as in ensureSymlink;
• a missing srcpath throws an ENOENT error with a code, instead of the plain "absolute/relative srcpath does not exist" message.

Nothing in the repo depends on the old messages. symlinkPathsSync is not exported.

Tests: added cases to lib/ensure/__tests__/symlink.test.js for a broken-symlink srcpath, both absolute and relative-to-cwd, and for the ENOENT code on a missing srcpath. All three fail without the change. npm test (standard, node test.js, node test.mjs): 748 passing.