feat: exposes context on mixed.test function and add originalValue to…

… context (#1021)

* adds this context as second argument and originalValute to ctx

* edits mixed.test function on the Readme

* fix: adjust readme and push code changes

README.md

@@ -706,26 +706,27 @@ use the alternate signature to provide more options (see below):
 let jimmySchema = string().test(
   'is-jimmy',
   '${path} is not Jimmy',
-  value => value === 'jimmy',
+  (value, context) => value === 'jimmy',
 );
 
 // or make it async by returning a promise
 let asyncJimmySchema = string().test(
   'is-jimmy',
   '${path} is not Jimmy',
-  async (value) => (await fetch('/is-jimmy/' + value)).responseText === 'true',
+  async (value, context) => (await fetch('/is-jimmy/' + value)).responseText === 'true',
 });
 
 await schema.isValid('jimmy'); // => true
 await schema.isValid('john'); // => false
 ```
 
-test functions are called with a special context, or `this` value, that exposes some useful metadata and functions. Note that to use the `this` context the test function must be a function expression (`function test(value) {}`), not an arrow function, since arrow functions have lexical context.
+test functions are called with a special context, or `this` value, that exposes some useful metadata and functions. Older versions just expose the `this` context using `function ()`, not arrow-func, but now it's exposed too as a second argument of the test functions. It's allow you decide which approach you prefer. 
 
 - `this.path`: the string path of the current validation
 - `this.schema`: the resolved schema object that the test is running against.
 - `this.options`: the `options` object that validate() or isValid() was called with
 - `this.parent`: in the case of nested schema, this is the value of the parent object
+- `this.originalValue`: the original value that is being tested
 - `this.createError(Object: { path: String, message: String, params: Object })`: create and return a
   validation error. Useful for dynamically setting the `path`, `params`, or more likely, the error `message`.
   If either option is omitted it will use the current path, or default message.

src/util/createValidation.js

@@ -44,12 +44,13 @@ export default function createValidation(config) {
       createError,
       resolve,
       options,
+      originalValue,
       ...rest,
     };
 
     if (!sync) {
       try {
-        Promise.resolve(test.call(ctx, value)).then((validOrError) => {
+        Promise.resolve(test.call(ctx, value, ctx)).then((validOrError) => {
           if (ValidationError.isError(validOrError)) cb(validOrError);
           else if (!validOrError) cb(createError());
           else cb(null, validOrError);
@@ -63,7 +64,7 @@ export default function createValidation(config) {
 
     let result;
     try {
-      result = test.call(ctx, value);
+      result = test.call(ctx, value, ctx);
 
       if (typeof result?.then === 'function') {
         throw new Error(