VC 2.0 add integration tests for language and base direction

[aljones15]

https://w3c.github.io/vc-data-model/#language-and-base-direction

When the language value object is used in place of a string value, the object MUST contain a @value property whose value is a string, and SHOULD contain a @language property whose value is a string containing a well-formed Language-Tag as defined by [BCP47], and MAY contain a @direction property whose value is a base direction string defined by the @direction attribute in [JSON-LD11]. The language value object MUST NOT include any other keys beyond @value, @language, and @direction.

Add tests to this library to ensure VC properties that support language and base direction work.

[aljones15]

• Branch off from the release candidate here: VC 2.0 Release Canidate #148
• add the tests as a separate PR on the candidate
• This might require changes in a lower level library such as jsonld-signatures or jsonld