[njs] Fixed TypeScript description for NjsByteString type.

[Dmitry Volyntsev]

details:   https://hg.nginx.org/njs/rev/f2e8486b3b6d
branches:  
changeset: 1449:f2e8486b3b6d
user:      Artem S. Povalyukhin <artem.povaluhin at gmail.com>
date:      Sat Jun 27 19:26:41 2020 +0300
description:
Fixed TypeScript description for NjsByteString type.

NjsByteString was declared as it has no relation to string primitive type.
This causes a problem when a variable of type NjsByteString is passed into
built-in functions expecting string type.

The issue was introduced in 8f3ef384f69e.

diffstat:

 src/ts/njs_core.d.ts |  10 +++++-----
 1 files changed, 5 insertions(+), 5 deletions(-)

diffs (36 lines):

diff -r 1c729f765cfb -r f2e8486b3b6d src/ts/njs_core.d.ts
--- a/src/ts/njs_core.d.ts	Thu Jul 02 14:00:16 2020 +0000
+++ b/src/ts/njs_core.d.ts	Sat Jun 27 19:26:41 2020 +0300
@@ -14,14 +14,14 @@ interface String {
      * Serializes a Unicode string with code points up to 255
      * into a byte string, otherwise, null is returned.
      */
-    toBytes(start?: number, end?: number): NjsByteString;
+    toBytes(start?: number, end?: number): NjsByteString | null;
     /**
      * Serializes a Unicode string to a byte string using UTF8 encoding.
      */
     toUTF8(start?: number, end?: number): NjsByteString;
 }
 
-interface NjsByteString extends String {
+type NjsByteString = string & {
     /**
      * Returns a new Unicode string from a byte string where each byte is replaced
      * with a corresponding Unicode code point.
@@ -31,12 +31,12 @@ interface NjsByteString extends String {
      * Converts a byte string containing a valid UTF8 string into a Unicode string,
      * otherwise null is returned.
      */
-    fromUTF8(start?: number, end?: number): string;
+    fromUTF8(start?: number, end?: number): string | null;
     /**
      * Encodes a byte string to hex, base64, or base64url.
      */
-    toString(encoding?: "hex" | "base64" | "base64url"): string;
-}
+    toString(encoding: "hex" | "base64" | "base64url"): string;
+};
 
 type NjsStringLike = string | NjsByteString;