[#1744] Fix document link on Windows

Motivation and Context

VSCode fails to open a document link on Windows

Description

Add file:// prefix to the target of document link: it's expected to be URI by LSP specification, but as in https://github.com/ocaml/ocaml-lsp/issues/1117 it has wrong type in ocaml-lsp.

Also capitalize the drive letter in document link target on Windows so link will point to e.g. C:... instead of c:...

Now document links are opened correctly

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Fixed LSP "document link" feature on Windows

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).