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/project/src/project.rs

@@ -10861,12 +10861,19 @@ impl Project {
         cx: &mut ModelContext<Self>,
     ) -> Task<Result<Vec<(TaskSourceKind, TaskTemplate)>>> {
         if self.is_local() {
-            let language = location
-                .and_then(|location| location.buffer.read(cx).language_at(location.range.start));
+            let (file, language) = location
+                .map(|location| {
+                    let buffer = location.buffer.read(cx);
+                    (
+                        buffer.file().cloned(),
+                        buffer.language_at(location.range.start),
+                    )
+                })
+                .unwrap_or_default();
             Task::ready(Ok(self
                 .task_inventory()
                 .read(cx)
-                .list_tasks(language, worktree)))
+                .list_tasks(file, language, worktree, cx)))
         } else if let Some(project_id) = self
             .remote_id()
             .filter(|_| self.ssh_connection_string(cx).is_some())