feat(task): introduce built-in unset command to deno task (#18606)

This introduces a new built-in `unset` command to `deno task` by bumping
deno_task_shell to the latest version 0.11.0. Also this includes a fix
on how `deno task` handles empty environment variables (see
denoland/deno_task_shell#80 for details).

Author: magurotuna

Cargo.lock

@@ -48,7 +48,7 @@ deno_graph = "=0.45.0"
 deno_lint = { version = "0.43.0", features = ["docs"] }
 deno_lockfile.workspace = true
 deno_runtime = { workspace = true, features = ["dont_create_runtime_snapshot", "include_js_files_for_snapshotting"] }
-deno_task_shell = "0.10.0"
+deno_task_shell = "0.11.0"
 napi_sym.workspace = true
 
 async-trait.workspace = true