Flags inside pragma

In `{-# OPTIONS_GHC -F -pgmF tasty-discover -optF --tree-display #-}`, the `--` is recognised as a comment, and so (at least with my current theme), the remainder of the line is greyed out.

I guess we shouldn't be looking for the single-line comment syntax while we're already inside a multi-line comment.

Reproduce the issue in VS Code using the OPTIONS_GHC pragma shown in the report, then inspect the extension's Haskell syntax-highlighting rules for pragma and comment handling. Done means the `--tree-display` argument is not treated as a single-line comment while inside the pragma.

Written by the indexing model from the issue text.