Docs: Add VS Code file association instructions for .posecode extension #20

We have a custom language spec for files, but until the full extension is published, VS Code users have to manually configure file associations.

We should add a quick section in the main README or a new editors/vscode/README.md showing how to add this setting to :

"files.associations": {
  "*.posecode": "markdown"
}

Tasks:

1. Document file association settings for VS Code.
2. Document how to configure Cursor/other editors if applicable.
3. Submit a PR.