feat(child_process): add exec and execFile functions

Implements exec() and execFile() for the child_process module, matching
the Node.js API. Key features:

- Callback-based API with (error, stdout, stderr) signature
- Shell execution for exec(), direct execution for execFile()
- Support for options: cwd, env, timeout, maxBuffer, killSignal, shell
- Preserves partial output data on timeout
- Uses Persistent<Function> pattern for async callback safety

Author: chessbyte

API.md

@@ -114,6 +114,10 @@ Everything else inherited from [Uint8Array](https://developer.mozilla.org/en-US/
 
 [spawn](https://nodejs.org/api/child_process.html#child_processspawncommand-args-options)
 
+[exec](https://nodejs.org/api/child_process.html#child_processexeccommand-options-callback)
+
+[execFile](https://nodejs.org/api/child_process.html#child_processexecfilefile-args-options-callback)
+
 ## console
 
 [Console](https://nodejs.org/api/console.html#class-console)

modules/llrt_child_process/Cargo.toml

@@ -20,10 +20,11 @@ llrt_utils = { version = "0.7.0-beta", path = "../../libs/llrt_utils", default-f
 rquickjs = { git = "https://github.com/DelSkayn/rquickjs.git", version = "0.10.0", features = [
     "std",
 ], default-features = false }
-tokio = { version = "1", features = ["process"], default-features = false }
+tokio = { version = "1", features = ["process", "time"], default-features = false }
 
 [target.'cfg(unix)'.dependencies]
 libc = { version = "0.2", default-features = false }
 
 [dev-dependencies]
 llrt_test = { path = "../../libs/llrt_test" }
+tokio = { version = "1", features = ["rt"] }