feat: pass logger/child logger as param to mixin

Passing the logger or child logger as a parameter to `mixin` allows
users to set logger-specific context in the logger object that can be
used by `mixin` to enrich the context to be added to the resulting JSON.

One example use case for this is avoiding the "duplicate keys" caveat
that comes with child loggers. If the user wants to make a known key
"mergeable", they could add a custom function that concatenates values
to the key and then use `mixin` to pass that merged value to the
context. For example:

```js
instance = pino({
  mixin(obj, num, logger) {
    return {
      "tags": logger.tags
    }
  }
})
instance.tags = {}

instance.addTag = function (key, value) {
  child.tags[key] = value
}

function createChild (logger, ...context) {
  const child = logger.child(...context)
  child.tags = { ...logger.tags }
  child.addTag = function (key, value) {
    child.tags[key] = value
  }
  return child
}

instance.addTag('foo', 1)
child.addTag('bar', 2)
instance.info('this will only have `foo: 1`')
child.info('this will have both `foo: 1` and `bar: 2`')
```

lib/proto.js

@@ -194,7 +194,7 @@ function write (_obj, msg, num) {
   }
 
   if (mixin) {
-    obj = mixinMergeStrategy(obj, mixin(obj, num))
+    obj = mixinMergeStrategy(obj, mixin(obj, num, this))
   }
 
   const s = this[asJsonSym](obj, msg, num, t)

test/mixin.test.js

@@ -160,3 +160,59 @@ test('mixin can use level number', async ({ ok, same }) => {
     stack: 'stack'
   }, 'test')
 })
+
+test('mixin receives logger as third parameter', async ({ ok, same }) => {
+  const stream = sink()
+  const instance = pino({
+    mixin (context, num, logger) {
+      ok(logger !== null, 'logger should be defined')
+      ok(logger !== undefined, 'logger should be defined')
+      same(logger, instance)
+      return { ...context, num }
+    }
+  }, stream)
+  instance.level = name
+  instance[name]({
+    message: '123'
+  }, 'test')
+})
+
+test('mixin receives child logger', async ({ ok, same }) => {
+  const stream = sink()
+  let child = null
+  const instance = pino({
+    mixin (context, num, logger) {
+      ok(logger !== null, 'logger should be defined')
+      ok(logger !== undefined, 'logger should be defined')
+      same(logger.expected, child.expected)
+      return { ...context, num }
+    }
+  }, stream)
+  instance.level = name
+  instance.expected = false
+  child = instance.child({})
+  child.expected = true
+  child[name]({
+    message: '123'
+  }, 'test')
+})
+
+test('mixin receives logger even if child exists', async ({ ok, same }) => {
+  const stream = sink()
+  let child = null
+  const instance = pino({
+    mixin (context, num, logger) {
+      ok(logger !== null, 'logger should be defined')
+      ok(logger !== undefined, 'logger should be defined')
+      same(logger.expected, instance.expected)
+      return { ...context, num }
+    }
+  }, stream)
+  instance.level = name
+  instance.expected = false
+  child = instance.child({})
+  child.expected = true
+  instance[name]({
+    message: '123'
+  }, 'test')
+})