Support syntax highlight for multiline using directives

[tgodzik]

Is your feature request related to a problem? Please describe.
Currently, https://github.com/scala/vscode-scala-syntax supports highlighting single line using directives, however that breaks when using multiline ones.

Describe the solution you'd like
Add support to vscode-scala-syntax for multiline using directives, it should enable it in vscode as well as in github.

Describe alternatives you've considered
No highlighting or using semantic highlighting

Additional context
Previous PR scala/vscode-scala-syntax#237

[lwronski]

I will close it because it is no longer valid. Using directives now is a simple key-value without multiline support.