RNA: allow updating textdomains in JS and PHP packages used in multiple plugins

[jeherve]

Our PHP and JS packages currently use the jetpack textdomain for all translatable strings. While that's the right thing to do for all strings that are shipped with the Jetpack plugin, those strings will not be translated if they are shipped as is in other plugins with a different slug.

We should consequently consider automatically updating those textdomains on production versions of each plugin.

• Update strings in JS packages
• Update strings in PHP packages

@kbrown9 looked into the PHP part of this last year, in #18070, and offered a solution that was added to our docs:

jetpack/projects/packages/README.md

Lines 64 to 82 in d6b0536

	### Package textdomains
	Jetpack's packages use the 'jetpack' textdomain for translatable strings. Plugins that use these packages must change the textdomains in the packages when preparing the plugin for release. A few tools that can help automate this process are [node-wp-i18n](https://github.com/cedaro/node-wp-i18n) and [wp-textdomain](https://github.com/timelsass/wp-textdomain).
	For example, a plugin could change the textdomain during the Composer `post-autoload-dump` event with the following script in the plugin's `composer.json` file:
	`"post-autoload-dump": "node {path_to_script}/update_textdomain.js"`
	where the `update_textdomain.js` file contains something like:
	```
	const wpTextdomain = require( 'wp-textdomain' );
	wpTextdomain( './vendor/automattic/**/*.php', {
	domain: 'plugin-textdomain',
	fix: true,
	glob: { follow: true },
	} );
	```

It may be time to start adding that solution to our monorepo plugin that consumes packages, the Backup plugin.

---

Related: #17099

---

Example use-case: Jetpack components (see #20287).

[anomiex]

I believe everything here has been done.

The JS part is done more or less as proposed here via the babel-plugin-replace-textdomain package, the domains are rewritten as the bundle is being produced.

For the PHP part, instead of rewriting packages when embedded into the plugins we did as described in #21690 (comment).