task: Add task contexts (#8675)

This PR supplements tasks with additional environment variables; ideally
we'll be able to write a task like:
`cargo test -p $ZED_CURRENT_PACKAGE -- $ZED_CURRENT_FUNCTION`
- [x] Flesh out multibuffer interactions
- [x] Add ZED_SYMBOL detection based on tree-sitter queries
- [ ] Add release note and demo
- [x] Figure out a solution for rerun dilemma - should `task: rerun`
reevaluate contexts for tasks?

This PR introduced the following variables:
- ZED_COLUMN - current line column
- ZED_ROW - current line row
and the following, which are available for buffers with associated
files:
- ZED_WORKTREE_ROOT - absolute path to the root of the current worktree.
- ZED_FILE - absolute path to the file
- ZED_SYMBOL - currently selected symbol; should match the last symbol
shown in a symbol breadcrumb (e.g. `mod tests > fn test_task_contexts`
should be equal to ZED_SYMBOL of `test_task_contexts`). Note that this
isn't necessarily a test function or a function at all.

Also, you can use them in `cwd` field of definitions (note though that
we're using https://docs.rs/subst/latest/subst/#features for that, so
don't expect a full shell functionality to work); the syntax should
match up with your typical Unix shell.


Release Notes:

- Added task contexts, which are additional environment variables set by
Zed for task execution; task content is dependent on the state of the
editor at the time the task is spawned.

---------

Co-authored-by: Anthony <anthonyeid7@protonmail.com>

crates/task/src/oneshot_source.rs

@@ -2,7 +2,7 @@
 
 use std::sync::Arc;
 
-use crate::{SpawnInTerminal, Task, TaskId, TaskSource};
+use crate::{SpawnInTerminal, Task, TaskContext, TaskId, TaskSource};
 use gpui::{AppContext, Context, Model};
 
 /// A storage and source of tasks generated out of user command prompt inputs.
@@ -30,21 +30,22 @@ impl Task for OneshotTask {
         &self.id.0
     }
 
-    fn cwd(&self) -> Option<&std::path::Path> {
+    fn cwd(&self) -> Option<&str> {
         None
     }
 
-    fn exec(&self, cwd: Option<std::path::PathBuf>) -> Option<SpawnInTerminal> {
+    fn exec(&self, cx: TaskContext) -> Option<SpawnInTerminal> {
         if self.id().0.is_empty() {
             return None;
         }
+        let TaskContext { cwd, env } = cx;
         Some(SpawnInTerminal {
             id: self.id().clone(),
             label: self.name().to_owned(),
             command: self.id().0.clone(),
             args: vec![],
             cwd,
-            env: Default::default(),
+            env,
             use_new_terminal: Default::default(),
             allow_concurrent_runs: Default::default(),
         })