Support .hs-boot files

[ice1000]

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At least for syntax highlight.

[niklasbroberg]

I second this. The language of hs-boot files is the same as for regular .hs file (or a subset rather), so this ought to be a quick fix. Is it as simple as adding another entry like this:

<fileType name="Haskell Boot File" language="Haskell" implementationClass="intellij.haskell.HaskellFileType" extensions="hs-boot" fieldName="INSTANCE"/>

to the plugins.xml file? I'd write a PR if I was sure I wasn't missing something important.

[ice1000]

You are! If you do this the parser will also be applied, and the code analysis will also be applied. I expect the language server to have some special support on hs-boot files :thinking: but most code should be reused for sure.