Add writing-mode as an allowed attribute to safecss_filter_attr()

[wildworks]

#59306 added writing-mode to the typography support. However, this property is not allowed by safecss_filter_attr() function, so this property will be removed when outputting inline styles with the get_block_wrapper_attributes() function, for example.

This ticket is for tracking the PHP backport for the following Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/54581

[mukesh27]

@wildworks thanks for ticket and patch.

Could you please document since annotation for the function?

* @since 6.4.0 Added support for `writing-mode`.

[wildworks]

Add @since annotation for the function and fix typo in comment text of phpunit test

[wildworks]

I submitted patch-1.diff with added @since annotation.

There was a typo in the comment text of the phpunit test, so I fixed it at the same time (// Wiritng mode introduced in 6.4. > // Writing mode introduced in 6.4.).

[SergeyBiryukov]

In 56617:

KSES: Add writing-mode to the list of safe CSS properties.

Original PR from Gutenberg repository:

• ​#54581 Gutenberg Plugin: Add hook to allow `writing-mode` as a safe CSS property

Reference: ​MDN Web Docs: writing-mode.

Follow-up to [56605].

Props wildworks, mukesh27, poena, andrewserong.
Fixes #59387.