API Added extension points to ShortcodeParser

[jonom]

Allows us to solve problems like sheadawson/silverstripe-shortcodable#8

[helpfulrobot]

@jonom, thanks for your PR! By analyzing the blame information on this pull request, I identified @sminnee, @dhensby and @chillu to be potential reviewers

[jonom]

To complement this change, might be good to change a lot of the protected stuff to public so that extensions can make use of it. In particular there's no way at the moment to get info on registered shortcodes, data is stored in protected $shortcodes = array() with no getter. You can see if a shortcode is registered with registered($shortcode) but can't look up the class and method responsible for rendering it.

Would there be any drawback to opening this stuff up?

[dhensby]

#5535 and #5987 are related.

I think adding some public getters makes sense and can go into the 3 branch too

[jonom]

@dhensby have added a getter for registered shortcodes which seems like the most useful thing to expose for now and doesn't mess with visibility of existing methods.

[jonom]

Thanks Dan