Add an ESM compatible build to grunt script via Rollup (#99)

* add rollup dev dependencies

* change export mechanism...

This does not change the `dist` command's current outputs

* add rollup es module bundling to grunt dist task

* Fix typo

* add "module" esm target file

* add built esm outputs

dist/esm/abs.d.ts

@@ -0,0 +1 @@
+export declare function abs(n: number | string | bigint): string;

dist/esm/add.d.ts

@@ -0,0 +1,3 @@
+export declare function add(number1: string, number2?: string): string;
+export declare function trim(number: string): string;
+export declare function pad(number1: string, number2: string): string[];

dist/esm/big-decimal.d.ts

@@ -0,0 +1,34 @@
+import { RoundingModes as Modes } from './roundingModes';
+declare class bigDecimal {
+    private value;
+    static RoundingModes: typeof Modes;
+    private static validate;
+    constructor(number?: (number | string | bigint));
+    getValue(): string;
+    setValue(num: (number | string | bigint)): void;
+    static getPrettyValue(number: any, digits: any, separator: any): string;
+    getPrettyValue(digits: any, separator: any): string;
+    static round(number: any, precision?: number, mode?: Modes): string;
+    round(precision?: number, mode?: Modes): bigDecimal;
+    static abs(number: any): string;
+    abs(): bigDecimal;
+    static floor(number: any): any;
+    floor(): bigDecimal;
+    static ceil(number: any): any;
+    ceil(): bigDecimal;
+    static add(number1: any, number2: any): string;
+    add(number: bigDecimal): bigDecimal;
+    static subtract(number1: any, number2: any): string;
+    subtract(number: bigDecimal): bigDecimal;
+    static multiply(number1: any, number2: any): string;
+    multiply(number: bigDecimal): bigDecimal;
+    static divide(number1: any, number2: any, precision: any): string;
+    divide(number: bigDecimal, precision: any): bigDecimal;
+    static modulus(number1: any, number2: any): string;
+    modulus(number: bigDecimal): bigDecimal;
+    static compareTo(number1: any, number2: any): 0 | 1 | -1;
+    compareTo(number: bigDecimal): 0 | 1 | -1;
+    static negate(number: any): string;
+    negate(): bigDecimal;
+}
+export default bigDecimal;