Change type of path.sep and path.delimiter in @types/node (#24833)

* Update type of path.sep and path.delimiter

* Change `path.sep` to `'\\' | '/'`
* Change `path.delimiter` to  `';' | ':'`

* Update index.d.ts

* Change types of path.sep and path.delimiter in types/node/{v8,v6}

* Change types of path.sep and path.delimiter in types/node/v7

types/node/index.d.ts

@@ -20,6 +20,7 @@
 //                 Klaus Meinhardt <https://github.com/ajafff>
 //                 Huw <https://github.com/hoo29>
 //                 Nicolas Even <https://github.com/n-e>
+//                 Hoàng Văn Khải <https://github.com/KSXGitHub>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 /** inspector module types */
@@ -4683,11 +4684,11 @@ declare module "path" {
     /**
      * The platform-specific file separator. '\\' or '/'.
      */
-    export var sep: string;
+    export var sep: '\\' | '/';
     /**
      * The platform-specific file delimiter. ';' or ':'.
      */
-    export var delimiter: string;
+    export var delimiter: ';' | ':';
     /**
      * Returns an object from a path string - the opposite of format().
      *

types/node/v6/index.d.ts

@@ -6,6 +6,7 @@
 //                 Thomas Bouldin <https://github.com/inlined>
 //                 Sebastian Silbermann <https://github.com/eps1lon>
 //                 Alorel <https://github.com/Alorel>
+//                 Hoàng Văn Khải <https://github.com/KSXGitHub>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 /************************************************
@@ -3003,11 +3004,11 @@ declare module "path" {
     /**
      * The platform-specific file separator. '\\' or '/'.
      */
-    export var sep: string;
+    export var sep: '\\' | '/';
     /**
      * The platform-specific file delimiter. ';' or ':'.
      */
-    export var delimiter: string;
+    export var delimiter: ';' | ':';
     /**
      * Returns an object from a path string - the opposite of format().
      *

types/node/v7/index.d.ts

@@ -6,6 +6,7 @@
 //                 Christian Vaagland Tellnes <https://github.com/tellnes>
 //                 Wilco Bakker <https://github.com/WilcoBakker>
 //                 Sebastian Silbermann <https://github.com/eps1lon>
+//                 Hoàng Văn Khải <https://github.com/KSXGitHub>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 /************************************************
@@ -3092,11 +3093,11 @@ declare module "path" {
     /**
      * The platform-specific file separator. '\\' or '/'.
      */
-    export var sep: string;
+    export var sep: '\\' | '/';
     /**
      * The platform-specific file delimiter. ';' or ':'.
      */
-    export var delimiter: string;
+    export var delimiter: ';' | ':';
     /**
      * Returns an object from a path string - the opposite of format().
      *

types/node/v8/index.d.ts

@@ -19,6 +19,7 @@
 //                 Alberto Schiabel <https://github.com/jkomyno>
 //                 Huw <https://github.com/hoo29>
 //                 Nicolas Even <https://github.com/n-e>
+//                 Hoàng Văn Khải <https://github.com/KSXGitHub>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.1
 
@@ -4662,11 +4663,11 @@ declare module "path" {
     /**
      * The platform-specific file separator. '\\' or '/'.
      */
-    export var sep: string;
+    export var sep: '\\' | '/';
     /**
      * The platform-specific file delimiter. ';' or ':'.
      */
-    export var delimiter: string;
+    export var delimiter: ';' | ':';
     /**
      * Returns an object from a path string - the opposite of format().
      *