Proper Syntax Highlighting for Fragment Variables

Apollo has experimental support for Fragment Variables. https://github.com/apollographql/graphql-tag#experimental-fragment-variables

It would be excellent if the `vscode-apollo` extension properly highlighted these fragments. Thanks! 😄

Start with the vscode-apollo extension and review the experimental Fragment Variables behavior documented by graphql-tag. Trace how the extension highlights GraphQL fragments, then verify that fragment variables receive proper syntax highlighting; the payload does not name specific files or tests.

Written by the indexing model from the issue text.