tasks: Add experimental support for user-defined task variables (#13699)

Context:
@bennetbo spotted a regression in handling of `cargo run` task in zed
repo following a merge of #13658. We've started invoking `cargo run`
from the folder of an active file whereas previously we did it from the
workspace root. We brainstormed few solutions that involved adding a
separate task that gets invoked at a workspace level, but I realized
that a cleaner solution may be to finally add user-configured task
variables. This way, we can choose which crate to run by default at a
workspace level.

This has been originally brought up in the context of javascript tasks
in
https://github.com/zed-industries/zed/pull/12118#issuecomment-2129232114

Note that this is intended for internal use only for the time being.
/cc @RemcoSmitsDev we should be unblocked on having runner-dependant
tasks now.

Release notes:

- N/A

crates/languages/src/go.rs

@@ -1,7 +1,7 @@
 use anyhow::{anyhow, Context, Result};
 use async_trait::async_trait;
 use futures::StreamExt;
-use gpui::{AsyncAppContext, Task};
+use gpui::{AppContext, AsyncAppContext, Task};
 use http::github::latest_github_release;
 pub use language::*;
 use lazy_static::lazy_static;
@@ -501,7 +501,11 @@ impl ContextProvider for GoContextProvider {
         ))
     }
 
-    fn associated_tasks(&self) -> Option<TaskTemplates> {
+    fn associated_tasks(
+        &self,
+        _: Option<Arc<dyn language::File>>,
+        _: &AppContext,
+    ) -> Option<TaskTemplates> {
         Some(TaskTemplates(vec![
             TaskTemplate {
                 label: format!(