Merge Zed task context providing logic (#10544)

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

crates/language/src/language.rs

@@ -56,7 +56,7 @@ use std::{
     },
 };
 use syntax_map::SyntaxSnapshot;
-pub use task_context::{ContextProvider, ContextProviderWithTasks, SymbolContextProvider};
+pub use task_context::{BasicContextProvider, ContextProvider, ContextProviderWithTasks};
 use theme::SyntaxTheme;
 use tree_sitter::{self, wasmtime, Query, WasmStore};
 use util::http::HttpClient;