Introduce class modification helpers for block markup

[kadamwhite]

Adds three new chaining methods to make it easier to update a class in both the block's HTML and the block's attributes simultaneously:

->add_class(
    pattern_slug: 'theme/hero',
    block_type:   'core/heading',
    occurrence:   0,
    classes:      'is-style-display'
)
->remove_class(
    'theme/hero',
    'core/paragraph',
    occurrence: 0,
    classes: 'is-style-lede'
)
->replace_class(
    pattern_slug: 'theme/hero',
    block_type:   'core/heading',
    occurrence:   0,
    old_classes:  'is-style-default',
    new_classes:  'is-style-display'
)

[kadamwhite]

Suggested change

	->remove_class( 'theme/hero', 'core/paragraph', occurrence: 0, classes: 'is-style-lede' )
	->remove_class(
	pattern_slug: 'theme/hero',
	block_type: 'core/paragraph',
	occurrence: 0,
	classes: 'is-style-lede'
	)

I suppose we should be consistent in our docs style...?

[kadamwhite]

Things I'd like to add / fix : That situation where we can end up with <p > (whitespace left over from tag processor) probably breaks block validation, and I don't like that addClass and removeClass always execute in that order rather than respecting call order. It shouldn't matter logically but feels like it could somehow be an issue?

Edit: Confirmed the trailing whitespace is digested totally OK by simple-html-tokenizer, so no issue there.

[kadamwhite]

Adjusted my other wishlist item. Ready for review!