Modifiers for counting matches in strings

Count when a string or reggae appears a certain amount of times, this can be nested like:

``` php
$this->assert($this->assert('foo foo', contains_string, 'foo'), appears_twice);
```

`appears twice` is a synonym for `equals 2`

Start by locating the assertion and modifier parsing entry points in the Concise repository, then inspect existing tests for string matching and comparison synonyms. Implement the requested nested counting behavior and verify that the shown example and the “appears twice” synonym are covered by tests.

Written by the indexing model from the issue text.