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/editor/src/editor.rs

@@ -8469,13 +8469,14 @@ impl Editor {
         runnable: &mut Runnable,
         cx: &WindowContext<'_>,
     ) -> Vec<(TaskSourceKind, TaskTemplate)> {
-        let (inventory, worktree_id) = project.read_with(cx, |project, cx| {
-            let worktree_id = project
+        let (inventory, worktree_id, file) = project.read_with(cx, |project, cx| {
+            let (worktree_id, file) = project
                 .buffer_for_id(runnable.buffer)
                 .and_then(|buffer| buffer.read(cx).file())
-                .map(|file| WorktreeId::from_usize(file.worktree_id()));
+                .map(|file| (WorktreeId::from_usize(file.worktree_id()), file.clone()))
+                .unzip();
 
-            (project.task_inventory().clone(), worktree_id)
+            (project.task_inventory().clone(), worktree_id, file)
         });
 
         let inventory = inventory.read(cx);
@@ -8485,7 +8486,12 @@ impl Editor {
             .flat_map(|tag| {
                 let tag = tag.0.clone();
                 inventory
-                    .list_tasks(Some(runnable.language.clone()), worktree_id)
+                    .list_tasks(
+                        file.clone(),
+                        Some(runnable.language.clone()),
+                        worktree_id,
+                        cx,
+                    )
                     .into_iter()
                     .filter(move |(_, template)| {
                         template.tags.iter().any(|source_tag| source_tag == &tag)