topics/external-editor guide

Writing the engine's Luau in an external editor (VS Code, Neovim, or any editor running the luau-lsp language server): lsp.exportDefinitions writes the engine's API as a Luau definitions file (zero.d.luau), a documentation file and a .luaurc, the files are copied to the editor's machine, and luau-lsp pointed at them autocompletes the engine's globals, checks each call against its signature, reports a misspelled member, and shows the engine's documentation on hover.

Surfaced through guides { path: "topics/external-editor" } and man topics/external-editor. The content lives in guide.md.