Before, `tasks_ui` set most of the context with `SymbolContextProvider`
providing the symbol data part of the context. Now, there's a
`BasicContextProvider` that forms all standard Zed context and it
automatically serves as a base, with no need for other providers like
`RustContextProvider` to call it as before.

Also, stop adding `SelectedText` task variable into the context for
blank text selection.

Release Notes:

- N/A