Add missing exception classes: TypeMismatchException, ImmutableException, ValidationException

Author: nejc

src/Exceptions/ImmutableException.php

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+<?php
+
+namespace Nejcc\PhpDatatypes\Exceptions;
+
+class ImmutableException extends \Exception {} 

src/Exceptions/TypeMismatchException.php

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+<?php
+
+namespace Nejcc\PhpDatatypes\Exceptions;
+
+class TypeMismatchException extends \Exception {} 

src/Exceptions/ValidationException.php

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+<?php
+
+namespace Nejcc\PhpDatatypes\Exceptions;
+
+class ValidationException extends \Exception {}