Support for backpack signature files (`.hsig`)

[TotallyNotChase]

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Is your enhancement request related to a problem? Please describe.

HLS currently does not seem to support backpack. .hsig files. When using with VSCode, I get a parse error in where in the signature declaration:

signature MySig where
                ^^^^^ parse error on input ‘where’

Furthermore, importing signature modules also produce an error:

import MySig
       ^^^^^ Could not load module ‘MySig’

cabal build seems to work fine.

Describe the solution you'd like

Support backpack signatures

Describe alternatives you've considered

Not using HLS with backpack

Additional context