Improve handling of digits in CamelCaseToSeparator filter

[weierophinney]

This PR aims to add the behaviour asked in #33

Since the reason behind using the CamelCaseSeparator is tokenizing strings when case is changed, usual expectation is beginning a new token when hitting a number or digit.

I will try to summarize both current and changed behaviour on numbers with some examples:

Original Text	Current	After this PR
Foo2016Bar	Foo2016-Bar	Foo-2016-Bar
March16Events	March16-Events	March-16-Events
DigitSuffix42	Digit-Suffix42	Digit-Suffix-42
42metersLong	42metersLong	42-meters-Long

Notice the last example: Beginning of the word with lowercase letter after 42 is also mimics a case-change. I'm still not sure is this a acceptable/desired behaviour or not.

The issue #33 is about the CamelCaseToDashFilter but since it derived from CamelCaseToSeparator like CamelCaseToUnderscore, this change will affect both childs of CamelCaseToSeparator. For this reason, I also added more tests to CamelCaseToDashTest.

After digging dozen of approaches including other platforms such as Java and C# I came up with this final patterns with positive lookbehind and lookaheads. (Achieving same effect with a single regex seems like impossible, at least I give up)

I'm also aware of changing an existing test is not a good practice. Since the expected behaviour is changed for digits, I forced into touching the CamelCaseToUnderscoreTest:

• Pa2_Title => Pa_2_Title
• Pa2a_Title => Pa_2a_Title

Altering the CamelCaseToUnderscoreTest seems like mandatory if the desired behaviour will be changed.

All tests are still green.

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Originally posted by @edigu at zendframework/zend-filter#45

[weierophinney]

I'm pushing this to 3.0 (which will happen this year, as we will be releasing new major versions of all components that set PHP 7.1 as the minimum supported version). While I agree it is the correct behavior, I worry that if users are expecting the current behavior currently, this would break their applications. Pushing it to a new major version allows them to upgrade at their own pace.

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Originally posted by @weierophinney at zendframework/zend-filter#45 (comment)