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

.zed/settings.json

@@ -19,6 +19,13 @@
     "JavaScript": {
       "tab_size": 2,
       "formatter": "prettier"
+    },
+    "Rust": {
+      "tasks": {
+        "variables": {
+          "RUST_DEFAULT_PACKAGE_RUN": "zed"
+        }
+      }
     }
   },
   "formatter": "auto",