feat: add support for ImportQualifiedPost extension

[ncaq]

I add support for the ImportQualifiedPost extension
to fix parse errors that occur when processing code that uses this GHC extension.
The extension enables writing import statements with qualified keyword after the module name
(e.g., import Data.List qualified as L instead of the traditional import qualified Data.List as L).

Motivation

When tools like the sandwich test framework
process Haskell code that uses the ImportQualifiedPost extension,
haskell-src-exts was unable to parse these imports correctly, resulting in errors.
This implementation allows code using this extension
to be properly parsed and processed by tools that depend on haskell-src-exts.

Changes

• Added ImportQualifiedPost to the list of known extensions in Extension.hs
• Updated the parser grammar to support post-qualified imports by adding a new optqualified_post rule
• Modified the ExactPrint module to correctly handle post-qualified imports by determining the position of the qualified keyword based on source locations
• Added test cases with golden files to verify correct parsing and exact printing

Known Issue

The pretty printer still converts post-qualified imports back to pre-qualified form
(e.g., import qualified Data.List as L)
because the ImportDecl data structure only stores whether an import is qualified or not,
but not whether the qualification is pre or post. This is a limitation of the current API design.

Future improvements could involve modifying the ImportDecl data structure to explicitly track qualification style,
but this would require API changes and is out of scope for this implementation.