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/languages/src/lib.rs

@@ -87,7 +87,7 @@ pub fn init(
                     Ok((
                         config.clone(),
                         load_queries($name),
-                        Some(Arc::new(language::SymbolContextProvider)),
+                        Some(Arc::new(language::BasicContextProvider)),
                     ))
                 },
             );
@@ -107,7 +107,7 @@ pub fn init(
                     Ok((
                         config.clone(),
                         load_queries($name),
-                        Some(Arc::new(language::SymbolContextProvider)),
+                        Some(Arc::new(language::BasicContextProvider)),
                     ))
                 },
             );