expansion: string @value must not get @language/@direction if type mapping exists

I ended up with a bug in my implementation where Step 5 of value expansion (https://www.w3.org/TR/json-ld11-api/#algorithm-5) always happened if the value was a string, even if a type mapping was present. The test suite lacks coverage for this case, so this bug went unnoticed for almost a year.

A simple reproducer:

{
  "@context": {
    "typed": { "@id": "http://ex/typed", "@type": "http://ex/dt" },
    "@language": "und"
  },
  "typed": "v"
}

Expected:

[
  {
    "http://ex/typed": [
      {
        "@type": "http://ex/dt",
        "@value": "v"
      }
    ]
  }
]

Due to my bug I ended up with an "@language": "und" on http://ex/typed as well.

The Pleroma/Akkoma lineage of fediverse instances, as well as Mobilizon, all smash a {"@language": "und"} in their context.

Start with the Step 5 value-expansion algorithm and the existing test suite for value expansion. Use the JSON-LD context and reproducer in the issue as the regression case, then verify that a string with a type mapping produces @type and @value without @language or @direction.