fix: warn when markdown link path casing doesn't match actual file on…

[vibhumeh]

Fixes #106

On case insensitive filesystems (macOS and Windows), workspace.stat()
succeeds even when path casing doesn't match the actual file on disk.
This means a link like text silently resolves
even when the real file is docs/whitepaper.pdf, then breaks when
deployed to a case-sensitive system like GitHub (Linux).

Changes:

• Adds link.file-path-casing-mismatch diagnostic code
• Adds validateFileLinksMarkdownCaseSensitive option to DiagnosticOptions
  (defaults to true, set to false to opt out)
• After a file is confirmed to exist, readDirectory() on the parent
  verifies the exact basename casing matches

Note: this behavior cannot be fully unit tested with InMemoryWorkspace
since it simulates a case-sensitive filesystem. The casing check only
triggers on macOS/Windows where workspace.stat() succeeds despite path
casing mismatches, as documented in the VS Code API.

[vibhumeh]

@microsoft-github-policy-service agree