[JS/TS] Modernize TypeScript / JavaScript flatbuffers support (#6095)

ts/long.ts

@@ -0,0 +1,23 @@
+export function createLong(low: number, high: number): Long {
+    return Long.create(low, high);
+}
+  
+export class Long {
+    static readonly ZERO = new Long(0, 0)
+    low: number
+    high: number
+    constructor(low: number, high: number) {
+        this.low = low | 0;
+        this.high = high | 0;
+    }
+    static create(low: number, high: number): Long {
+        // Special-case zero to avoid GC overhead for default values
+        return low == 0 && high == 0 ? Long.ZERO : new Long(low, high);
+    }
+    toFloat64(): number {
+        return (this.low >>> 0) + this.high * 0x100000000;
+    }
+    equals(other: Long): boolean {
+        return this.low == other.low && this.high == other.high;
+    }
+}