Support WorkspaceSymbol in Flow Language Server

[jvilk-stripe]

Proposal

This is a feature request. I'd like for Flow's language server to support the WorkspaceSymbol request, which lets users search for where symbols are defined.

Use case

• User wants to import the FooBar React component, but doesn't know where it is defined.
• User uses the "Go to symbol in Workspace..." functionality in VS Code, and types in FooBar.
• VS Code sends a WorkspaceSymbol request to Flow's language server with FooBar.
• Flow's language server returns where FooBar is defined, which VS Code uses to bring the user to the definition site.