Replace `uriToFilePath` with vscode-uri

[nschonni]

VSCode hints you to do this https://github.com/microsoft/vscode-uri

let Files.uriToFilePath: (uri: string) => string
@deprecated
Use the vscode-uri npm module which provides a more complete implementation of handling VS Code URIs.

[coliff]

We no longer use uriToFilePath anywhere, but I did find that we're doing manual URI to file path conversion in a few places in server.ts.

The changes are much cleaner and more reliable now. The vscode-uri package handles all the edge cases for us, including:

• Proper URI parsing
• URL decoding
• Platform-specific path handling
• Proper handling of file:// protocol