Document expectations around filename casing

There have been many bugs relating to casing of filenames/paths in VS Code and language extensions in the past, so I think it's important that LSP defines expectations to ensure clients and servers have the same expectations so they work together correctly.

VS Code no longer uses the filesystem casing of a file in its APIs if the file is renamed, which means servers must be prepared to get different (the old) casing from VS Code after a file is renamed (see https://github.com/microsoft/vscode/issues/121106).

For example:

// File is open and named DANNY.dart
==> {"jsonrpc":"2.0","id":3,"method":"textDocument/documentSymbol","params":{"textDocument":{"uri":"file:///Users/danny/Desktop/casting_testing/bin/DANNY.dart"}}}

// File is renamed by user to danny.dart
==> {"jsonrpc":"2.0","id":4,"method":"workspace/willRenameFiles","params":{"files":[{"oldUri":"file:///Users/danny/Desktop/casting_testing/bin/DANNY.dart","newUri":"file:///Users/danny/Desktop/casting_testing/bin/danny.dart"}]}}

// VS Code/LSP continues to use the original casing
==> {"jsonrpc":"2.0","id":5,"method":"textDocument/documentSymbol","params":{"textDocument":{"uri":"file:///Users/danny/Desktop/casting_testing/bin/DANNY.dart"}}}

Many tools treat file paths case-sensitively even when the underlying filesystem is case-insensitive. In some cases (for example virtual workspaces) it might not even be possible to tell whether the underlying filesystem is case-sensitive, so it seems like servers might be forced to treat things all as case-insensitive.

It seems like the spec should have some guidelines/notes on this since it might not otherwise be obvious to implementers what they should do here (and depending on the environments they developer/test in, they may not see the same behaviour as other users).

Start by reviewing the LSP specification and the cited VS Code issue, then trace the textDocument/documentSymbol and workspace/willRenameFiles examples in this report. Document clear expectations for URI and filename casing across renames and filesystem environments, with guidance that lets clients and servers interoperate consistently.