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

assets/settings/default.json

@@ -128,7 +128,14 @@
   // The default number of lines to expand excerpts in the multibuffer by.
   "expand_excerpt_lines": 3,
   // Globs to match against file paths to determine if a file is private.
-  "private_files": ["**/.env*", "**/*.pem", "**/*.key", "**/*.cert", "**/*.crt", "**/secrets.yml"],
+  "private_files": [
+    "**/.env*",
+    "**/*.pem",
+    "**/*.key",
+    "**/*.cert",
+    "**/*.crt",
+    "**/secrets.yml"
+  ],
   // Whether to use additional LSP queries to format (and amend) the code after
   // every "trigger" symbol input, defined by LSP server capabilities.
   "use_on_type_format": true,
@@ -666,6 +673,10 @@
     // "max_scroll_history_lines": 10000,
   },
   "code_actions_on_format": {},
+  /// Settings related to running tasks.
+  "tasks": {
+    "variables": {}
+  },
   // An object whose keys are language names, and whose values
   // are arrays of filenames or extensions of files that should
   // use those languages.