Fix definitions highlighting after field and variable

[Hellzed]

Before this change, description strings are not highlighted after:

• the first field in a type,
• the first variable in a field.

Descriptions for type fields and field variables are explicitly allowed by the GraphQL spec.

This change fixes highlighting of string values in types and fields, according to the spec.

[FallingSnow]

Any chance of implementing this pull request?

[Hellzed]

@FallingSnow Actually I think it might cause other issues... I think it would just be better to port it to a TreeSitter grammar.