Vscode extension

[Ultra-MG]

This PR adds the initial OptiTrust VS Code extension and integrates it with the in-tree OptiTrust workflow.

Main features:

• Detect OptiTrust workspaces and project roots.
• Run OptiTrust view commands from VS Code.
• Add a dedicated OptiTrust output panel.
• Support step diff, full trace, step trace, and redo-last-view commands.
• Support running the current test and rerunning previous tests.
• Add default configurable keybindings.
• Add associated-file discovery with a top-right editor button.
• Open generated/expected files and compare outputs.
• Register .opti as the OptiLambda file extension.
• Add OptiLambda syntax highlighting and language configuration.
• Display generated diff and trace HTML inside VS Code webviews.
• Support C/C++ and OptiLambda representation switching in diff and trace views.
• Support OptiLambda Surface, Internal, and Fully-Typed representations.
• Add a health check for required tools and backend build status.
• Add README documentation and .vsix packaging support.

Notes:

• The extension does not replace OCaml-LSP.
• .ml files remain handled by the normal OCaml extension.
• .opti remains one language registration.
• C/C++ remains the default view.