fix: Make Sort.field optional (#78)

- Annotate `field` parameter as optional in docs and typings.
-  Set `field` on class instance only if it is not nil.

src/core/sort.js

@@ -31,19 +31,19 @@ const invalidUnitParam = invalidParam(ES_REF_URL, 'unit', UNIT_SET);
  *     .query(esb.termQuery('user', 'kimchy'))
  *     .sort(esb.sort('post_date', 'asc'))
  *
- * @param {string} field The field to sort on
+ * @param {string=} field The field to sort on.
+ * If a script is used to specify the sort order, `field` should be omitted.
  * @param {string=} order The `order` option can have the following values.
  * `asc`, `desc` to sort in ascending, descending order respectively.
  */
 class Sort {
     // eslint-disable-next-line require-jsdoc
     constructor(field, order) {
-        this._field = field;
-
         this._opts = {};
         this._geoPoint = null;
         this._script = null;
 
+        if (!isNil(field)) this._field = field;
         if (!isNil(order)) this.order(order);
     }
 

src/index.d.ts

@@ -8258,13 +8258,14 @@ declare namespace esb {
     /**
      * Allows creating and configuring sort on specified field.
      *
-     * @param {string} field The field to sort on
+     * @param {string=} field The field to sort on.
+     * If a script is used to specify the sort order, `field` should be omitted.
      * @param {string=} order The `order` option can have the following values.
      * `asc`, `desc` to sort in ascending, descending order respectively.
      */
     export class Sort {
-        constructor(field: string, order?: string);
-
+        constructor(field?: string, order?: string);
+        
         /**
          * Set order for sorting. The order defaults to `desc` when sorting on the `_score`,
          * and defaults to `asc` when sorting on anything else.
@@ -8417,11 +8418,12 @@ declare namespace esb {
     /**
      * Allows creating and configuring sort on specified field.
      *
-     * @param {string} field The field to sort on
+     * @param {string=} field The field to sort on.
+     * If a script is used to specify the sort order, `field` should be omitted.
      * @param {string=} order The `order` option can have the following values.
      * `asc`, `desc` to sort in ascending, descending order respectively.
      */
-    export function sort(field: string, order?: string): Sort;
+    export function sort(field?: string, order?: string): Sort;
 
     /**
      * A `rescore` request can help to improve precision by reordering just