Editor: KSES: Include Flex properties in CSS whitelist to support core Column block width

[aduth]

Related: #46597 (r45242)
Related: ​https://github.com/WordPress/gutenberg/pull/15499

As of ​Gutenberg 5.7, the Column block supports a width attribute which is implemented by applying a flex-basis inline style to the block's produced markup. This style attribute is not currently included in the `safe_style_css` filtered array and thus would be removed by users not privileged with the ​`unfiltered_html` capability (​filtered by Gutenberg). As was done with #46597, the default filtered array should be modified to include this new style attribute. This is only necessary if and when the Column enhancements arrive in trunk.

It is an open question whether all (or common) flex attributes should be whitelisted. In my initial patch, I will propose the inclusion of the minimal set necessary for Column block support.

[peterwilsoncc]

To ensure flex-basis is fully supported, let's add the related and shorthand properties as part of this ticket:

• flex
• flex-shrink
• flex-grow

All of these can be considered safe.

[aduth]

Thanks for the feedback @peterwilsoncc . I agree. The updated patch 47281.2.patch​ includes ​all properties of the `flex` shorthand.

[peterwilsoncc]

Added some tests in 47281.diff​, the main purpose is flex-shrink: 0 to ensure a bug doesn't get introduced that deletes falsey values.

[peterwilsoncc]

In 45363:

KSES: Add flex and related long form properties to safe CSS.

Allow flex, flex-grow, flex-shrink and flex-basis to be used in inline CSS. As of WordPress 5.3 the block editor is expected to use flex-basis inline to set the width in the column block.

Props aduth.
Fixes #47281.
See #37248.