Add support for F# open static class

[MangelMaxime]

Hello,

Disclaimer: I am new to Go, so I am unsure if what I did is all that is needed

In F# it is now possible to open static class using open type syntax. F# documentation

This PR tries to add support for this syntax.

In the test code, that I used I wanted to check that it the highlith worked for both top level instruction and instructions indented inside of a module.