Fix parsing of scoped package names in jest dependencies

[jtbandes]

The jest special parser did not properly recognize scoped dependencies, for example:

{
  transform: {
    '^.+\\.js$': '@swc/jest'
  }
}

The parser would strip off the /jest and was left only with @swc, which was then discarded. This change fixes the parsing to correctly strip off subdirectories (like @foo/bar/baz -> @foo/bar) accounting for both scoped and non-scoped package names.