Feature request: Unmark Context as internal

[jakubboucek]

Class \Nette\Schema\Context is marked as @internal.

schema/src/Schema/Context.php

Lines 15 to 18 in febf71f

	/**
	* @internal
	*/
	final class Context

It causes that I cannot make custom validator object because the ->normalize() method requires Context instance in argument declared by \Nette\Schema\Schema interface.

PhpStorm is here firing inspection warning:

Class 'Context' is marked as @internal  
Inspection info: Reports the usages of the entities that are marked as @internal.
In most cases, such usages should be removed or replaced with other constructs.

In addition Context is not fully internal, because custom validators need it for for collecting validation errors. I didn't found possibility to do it with more clean way.

Example of use:
https://github.com/redbitcz/subreg-api-php/blob/60f377ac68f3c1871b926eca336f5eb8d3368455/src/Schema/DateTime.php#L44-L64

Suggest to feature

Unmark \Nette\Schema\Context as internal.

I understand the Context class has currently too open public interface which is potentially sensitive. Maybe move collecting errors to more specifics object which will not be @internal marked.