Type spawn's args as a ReadonlyArray of string (#25487)

At the moment, this snippet:

```ts
const spawnArgs: ReadonlyArray<string> = ['foo', 'bar'];
spawn('foobar', spawnArgs);
```

fails with:

```
Argument of type 'ReadonlyArray<string>' is not assignable to parameter of type 'string[] | undefined'.
  Type 'ReadonlyArray<string>' is not assignable to type 'string[]'.
```

Looking at https://github.com/nodejs/node/blob/master/lib/child_process.js#L514-L535 and https://github.com/nodejs/node/blob/master/lib/internal/child_process.js#L286, it _looks_ like there is no mutation of the array. I am no expert so I'd be happy to be proven wrong, I just couldn't find sufficient evidence that a `ReadonlyArray` wouldn't be fine.
This is not a breaking change because passing a `string[]` is still valid.

types/node/index.d.ts

@@ -2101,7 +2101,7 @@ declare module "child_process" {
         windowsHide?: boolean;
     }
 
-    export function spawn(command: string, args?: string[], options?: SpawnOptions): ChildProcess;
+    export function spawn(command: string, args?: ReadonlyArray<string>, options?: SpawnOptions): ChildProcess;
 
     export interface ExecOptions {
         cwd?: string;

types/node/v0/index.d.ts

@@ -881,7 +881,7 @@ declare module "child_process" {
         unref(): void;
     }
 
-    export function spawn(command: string, args?: string[], options?: {
+    export function spawn(command: string, args?: ReadonlyArray<string>, options?: {
         cwd?: string;
         stdio?: any;
         custom?: any;

types/node/v4/index.d.ts

@@ -1240,7 +1240,7 @@ declare module "child_process" {
         gid?: number;
         shell?: boolean | string;
     }
-    export function spawn(command: string, args?: string[], options?: SpawnOptions): ChildProcess;
+    export function spawn(command: string, args?: ReadonlyArray<string>, options?: SpawnOptions): ChildProcess;
 
     export interface ExecOptions {
         cwd?: string;

types/node/v6/index.d.ts

@@ -1787,7 +1787,7 @@ declare module "child_process" {
         gid?: number;
         shell?: boolean | string;
     }
-    export function spawn(command: string, args?: string[], options?: SpawnOptions): ChildProcess;
+    export function spawn(command: string, args?: ReadonlyArray<string>, options?: SpawnOptions): ChildProcess;
 
     export interface ExecOptions {
         cwd?: string;

types/node/v7/index.d.ts

@@ -1847,7 +1847,7 @@ declare module "child_process" {
         gid?: number;
         shell?: boolean | string;
     }
-    export function spawn(command: string, args?: string[], options?: SpawnOptions): ChildProcess;
+    export function spawn(command: string, args?: ReadonlyArray<string>, options?: SpawnOptions): ChildProcess;
 
     export interface ExecOptions {
         cwd?: string;

types/node/v8/index.d.ts

@@ -2091,7 +2091,7 @@ declare module "child_process" {
         windowsHide?: boolean;
     }
 
-    export function spawn(command: string, args?: string[], options?: SpawnOptions): ChildProcess;
+    export function spawn(command: string, args?: ReadonlyArray<string>, options?: SpawnOptions): ChildProcess;
 
     export interface ExecOptions {
         cwd?: string;

types/node/v9/index.d.ts

@@ -2091,7 +2091,7 @@ declare module "child_process" {
         windowsHide?: boolean;
     }
 
-    export function spawn(command: string, args?: string[], options?: SpawnOptions): ChildProcess;
+    export function spawn(command: string, args?: ReadonlyArray<string>, options?: SpawnOptions): ChildProcess;
 
     export interface ExecOptions {
         cwd?: string;