Add message for GHC-87491

Author: RaoulHC

message-index/messages/GHC-87491/example1/after/ImportQualifiedPost.hs

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+{-# LANGUAGE Haskell2010 #-}
+{-# LANGUAGE ImportQualifiedPost #-}
+
+module ImportQualifiedPost where
+
+import Prelude qualified

message-index/messages/GHC-87491/example1/before/ImportQualifiedPost.hs

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+{-# LANGUAGE Haskell2010 #-}
+
+module ImportQualifiedPost where
+
+import Prelude qualified

message-index/messages/GHC-87491/example1/index.md

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+---
+title: Found 'qualified'  after the module
+---
+
+This file is written in `Haskell2010` which doesn't have the
+`ImportQualifiedPost` extension. To fix this you need to either turn on
+`ImportQualifiedPost`, or change the `qualified` keyword to be prefix.
+
+## Error Message
+```
+ImportQualifiedPost:5:16: error: [GHC-87491]
+    Found `qualified' in postpositive position.
+    Suggested fix: Perhaps you intended to use ImportQualifiedPost
+  |
+5 | import Prelude qualified
+  |                ^^^^^^^^^
+```

message-index/messages/GHC-87491/index.md

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+---
+title: Found 'qualified' after the module
+summary: qualified after the module requires the ImportQualifiedPost extension.
+severity: error
+introduced: 8.10.1
+---
+
+The [`ImportQualifiedPost extension`](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/import_qualified_post.html),
+allows you to put the `qualified` keyword after the module in imports, without
+this it must be before the module.
+This is part of the [`GHC2021`](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/control.html#extension-GHC2021)
+extension set.