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>

Cargo.lock

@@ -9467,6 +9467,16 @@ dependencies = [
  "syn 2.0.48",
 ]
 
+[[package]]
+name = "subst"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ca1318e5d6716d6541696727c88d9b8dfc8cfe6afd6908e186546fd4af7f5b98"
+dependencies = [
+ "memchr",
+ "unicode-width",
+]
+
 [[package]]
 name = "subtle"
 version = "2.5.0"
@@ -9698,6 +9708,7 @@ dependencies = [
  "schemars",
  "serde",
  "serde_json_lenient",
+ "subst",
  "util",
 ]
 
@@ -9716,6 +9727,8 @@ dependencies = [
  "serde",
  "serde_json",
  "task",
+ "tree-sitter-rust",
+ "tree-sitter-typescript",
  "ui",
  "util",
  "workspace",