Editors
Flow's editor integration is provided through the Language Server Protocol (LSP). The flow lsp command starts Flow as an LSP server, so any editor with an LSP client can connect to it for diagnostics, autocomplete, go-to-definition, hover types, code actions, and more.
Visual Studio Code
The officially maintained extension is Flow Language Support. Install it from the VS Code extensions panel or the Marketplace.
Other editors
Any editor with LSP support can be configured to use Flow by pointing its LSP client at flow lsp. Consult your editor or LSP plugin's documentation for setup details.