Add language toolchains (#19576)

This PR adds support for selecting toolchains for a given language (e.g.
Rust toolchains or Python virtual environments) with support for SSH
projects provided out of the box. For Python we piggy-back off of
[PET](https://github.com/microsoft/python-environment-tools), a library
maintained by Microsoft.
Closes #16421
Closes #7646

Release Notes:

- Added toolchain selector to the status bar (with initial support for
Python virtual environments)

crates/zed/src/zed.rs

@@ -208,6 +208,8 @@ pub fn initialize_workspace(
             activity_indicator::ActivityIndicator::new(workspace, app_state.languages.clone(), cx);
         let active_buffer_language =
             cx.new_view(|_| language_selector::ActiveBufferLanguage::new(workspace));
+        let active_toolchain_language =
+            cx.new_view(|cx| toolchain_selector::ActiveToolchain::new(workspace, cx));
         let vim_mode_indicator = cx.new_view(vim::ModeIndicator::new);
         let cursor_position =
             cx.new_view(|_| go_to_line::cursor_position::CursorPosition::new(workspace));
@@ -216,6 +218,7 @@ pub fn initialize_workspace(
             status_bar.add_left_item(activity_indicator, cx);
             status_bar.add_right_item(inline_completion_button, cx);
             status_bar.add_right_item(active_buffer_language, cx);
+                        status_bar.add_right_item(active_toolchain_language, cx);
             status_bar.add_right_item(vim_mode_indicator, cx);
             status_bar.add_right_item(cursor_position, cx);
         });