Align Flow lib defs for Node.js child_process with v24 (#55186)

Summary:
Pull Request resolved: #55186

This is an AI-assisted change to align the Flow definitions for the `child_process` module with the Node.js docs as at v24.

**New v18+ Features:**

1. **AbortSignal Support** (v18.0.0) - Process cancellation
   - Added `signal?: AbortSignal` option to all async functions (exec, execFile, fork, spawn)
   - Enables cancellation of child processes using `AbortController`
   - Example: `const controller = new AbortController(); spawn('cmd', [], {signal: controller.signal}); controller.abort();`
   - https://nodejs.org/api/child_process.html#child_processexeccommand-options-callback

2. **Enhanced Process Properties** (v20.x)
   - `spawnargs: Array<string>` - Full command-line arguments used to launch the process
   - `spawnfile: string` - Executable file name of the child process
   - `signalCode: string | null` - Signal that terminated the child process
   - https://nodejs.org/api/child_process.html#subprocessspawnargs

**ChildProcess Class Updates:**

3. **Improved Property Types**
   - `stdio` is now a properly typed tuple array with readonly modifier
   - `channel` changed from `Object` to `mixed` for better type safety

4. **Enhanced Method Signatures**
   - `kill(signal?: string | number): boolean` - Now returns boolean and accepts numeric signals
   - `send()` overloads properly typed with `child_process$Serializable` messages
   - `send()` accepts `keepOpen` option: `Readonly<{keepOpen?: boolean}>`
   - https://nodejs.org/api/child_process.html#subprocesskillsignal

**Option Type Improvements:**

6. **Modern Readonly Syntax** - All input options use `Readonly<{...}>`
   - `execOpts`, `execFileOpts`, `forkOpts`, `spawnOpts`, `spawnSyncOpts`, etc.
   - Allows passing readonly types safely

7. **Enhanced Environment Variables**
   - Changed `env?: Object` to `env?: {[key: string]: string | void}`
   - More precise typing for environment variable dictionaries

8. **New Fork Options** (IPC enhancements)
   - `serialization?: 'json' | 'advanced'` - IPC serialization format
   - `killSignal` and `timeout` options added to fork
   - https://nodejs.org/api/child_process.html#child_processforkmodulepath-args-options

9. **Enhanced Spawn Options**
   - Added `killSignal`, `timeout`, `serialization` to `spawnOpts`
   - Better consistency across all spawn functions

**Return Type Fixes:**

10. **spawnRet/spawnSyncRet** - Made fields properly nullable:
    - `status: number | null` - null if process never exited
    - `signal: string | null` - null if not terminated by signal
    - `error: Error | void` - void if no error occurred
    - Removed spread operator for exact types

**Helper Types:**

11. **`child_process$Serializable`** - Union type for IPC messages:
    - `string | number | boolean | bigint | {...} | Array<mixed>`
    - Used in send() method signatures

12. **`child_process$SendHandle`** - Type for handle passing:
    - `net$Server | net$Socket`
    - Used for passing server/socket handles to child processes

**References:**
- Node.js child_process module docs: https://nodejs.org/api/child_process.html
- AbortSignal integration: https://nodejs.org/api/child_process.html#child_processexeccommand-options-callback

Changelog: [Internal]
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Reviewed By: vzaidman

Differential Revision: D89944331

fbshipit-source-id: 744ae04051c2214d97269c48c115060f5f30fb1f

Author: robhogan