[@wordpress packages] Make the safeguard for using private APIs opt-in rather than opt-out

[zieladam]

Summary: The current system of sharing private APIs in Gutenberg (WordPress/gutenberg#46131) includes a series of safeguards: Only core JavaScript modules can participate in the private APIs system and each of them can only be registered once.

The latter safeguard is enabled by default in WordPress 6.2. However, in the most recent editor version it is disabled by default (WordPress/gutenberg#48352) and needs to be explicitly enabled by setting the IS_WORDPRESS_CORE to true.

## Description

Gutenberg introduced a system of sharing private APIs in
​https://github.com/WordPress/gutenberg/pull/46131. One of the
safeguards is a check preventing the same module from opting-in
twice so that contributors cannot easily gain access by pretending
to be a core module.

That safeguard is only meant for WordPress core and not for
the released @wordpress packages. However, right now it is
opt-out and must be explicitly disabled by developers wanting to
install the @wordpress packages. Let's make it opt-out instead.

This commit opts-out from that check in WordPress core by setting
the ALLOW_EXPERIMENT_REREGISTRATION to false. Once it's merged,
the Gutenberg plugin should be adjusted to use true as the
default value.

## Test plan:

Run npm run build with this PR applied. The resulting build/wp-includes/js/dist/js/private-apis.js file should say _process$env$ALLOW_EX = false.

Trac ticket: https://core.trac.wordpress.org/ticket/57795

cc @gziolo @ntsekouras @hellofromtonya

[hellofromTonya]

The change is for webpack as part of the build tooling. Though not technically a gutenberg-merge, it is part of ​Gutenberg PR 46131 scope. So tracking synchronization of changes from Gutenberg into Core, adding the gutenberg-merge keyword.

@hellofromtonya @peterwilsoncc let me know if you think this is suitable for merging, and if yes then I could make it my first commit.

[hellofromTonya]

Marking for commit. Assigning to @zieladam for commit.

@adamziel I marked the ticket for commit and assigned the commit to you. Go ahead with your first commit 🎉

[costdev]

As we're about to enter 6.2 RC, moving this ticket 6.3.

[zieladam]

In 55512:

Build/Test tools: Prevent registering the same private JavaScript API twice.

This sets the IS_WORDPRESS_CORE global variable to true in the webpack build script to enable extra safeguards around private JavaScript APIs.

Props costdev, hellofromTonya, gziolo, peterwilsoncc.
Fixes #57795.

[zieladam]

I just committed this to trunk – props to @gziolo for helping me during my first commit! This doesn't require a backport to WordPress 6.2 as it doesn't have any effects yet. It will only affect the build after the next editor merge.

Committed in https://core.trac.wordpress.org/changeset/55512/trunk – thank you everyone! 🎉