i18n: the isRTL flag should be provided by the current locale data

[jsnajdr]

Currently, when Gutenberg code needs to know if the current text direction is LTR or RTL, it gets it from various sources:

• the Block Editor has isRTL value in its settings
• components query the DOM: document.documentElement.dir === 'rtl'

But there is one place that could and should be the source of truth for everyone -- the @wordpress/i18n library and its current locale.

Every WordPress locale has a special translation string called _x( 'ltr', 'text direction' ). If the translation is rtl, then we're dealing with a RTL language. See the usage of this translation string in Core PHP WP_Locale class

@wordpress/i18n should export a new function that returns authoritative value of the RTL flag:

import { isRTL } from `@wordpress/i18n`;

const popoverAlignment = isRTL() : 'left' : 'right';

[sirreal]

I've opened #20298 to implement isRTL.

[sirreal]

With #20298 and #27838 I believe this is complete.