Make TypeScript recognize 'unreachable()' as an end point (AssemblyScript#1011)

Author: vladimir-tikhonov

NOTICE

@@ -20,6 +20,7 @@ under the licensing terms detailed in LICENSE:
 * Jay Phelps <hello@jayphelps.com>
 * jhwgh1968 <jhwgh1968@protonmail.com>
 * Jeffrey Charles <jeffreycharles@gmail.com>
+* Vladimir Tikhonov <reg@tikhonov.by>
 
 Portions of this software are derived from third-party works licensed under
 the following terms:

std/assembly/index.d.ts

@@ -107,8 +107,8 @@ declare function trunc<T = f32 | f64>(value: T): T;
 declare function load<T>(ptr: usize, immOffset?: usize, immAlign?: usize): T;
 /** Stores a value of the specified type to memory. Equivalent to dereferencing a pointer in other languages when assigning a value. */
 declare function store<T>(ptr: usize, value: any, immOffset?: usize, immAlign?: usize): void;
-/** Emits an unreachable operation that results in a runtime error when executed. Both a statement and an expression of any type. */
-declare function unreachable(): any; // sic
+/** Emits an unreachable operation that results in a runtime error when executed. Both a statement and an expression. */
+declare function unreachable(): never;
 
 /** NaN (not a number) as a 32-bit or 64-bit float depending on context. */
 declare const NaN: f32 | f64;