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/language/src/task_context.rs

@@ -1,4 +1,4 @@
-use std::ops::Range;
+use std::{ops::Range, sync::Arc};
 
 use crate::{Location, Runnable};
 
@@ -31,7 +31,11 @@ pub trait ContextProvider: Send + Sync {
     }
 
     /// Provides all tasks, associated with the current language.
-    fn associated_tasks(&self) -> Option<TaskTemplates> {
+    fn associated_tasks(
+        &self,
+        _: Option<Arc<dyn crate::File>>,
+        _cx: &AppContext,
+    ) -> Option<TaskTemplates> {
         None
     }
 }