Unhide the classic menu widget

[draganescu]

Description

According to the Track ticket 53301

The legacy "Navigation Menu" widget is missing from the block-widget inserter

All the classic widgets have been replaced by blocks and hence the decision to hide them from being used makes sense. However the "Navigation Menu" widget is not fully replaced by the Navigation because the Navigation block is currently missing the ability to update itself if a classic menu has been changed. The Navigation block can only import a classic menu, creating a static copy of it. This behavior is different and unexpected in the widgets context.

While the LegacyWidget block does display the "Navigation Menu" widget, adding new ones is not possible without this change. It should be possible, considering we don't even have a transform yet from the "Navigation Menu" widget to the Navigatioin block, liike all the other core classic widgets do.

How has this been tested?

1. With Gutenberg trunk active
2. Go to Appearance > Widgets
3. Open the inserter from the top blue plus button
4. Insert a "Navigation Menu" from the "Widgets" section
5. It should work.

Screenshots

Types of changes

Removed the nav_menu widget from the widget_types_to_hide_from_legacy_widget_block filter.
Also fixed a typo in a comment.

[noisysocks]

This looks good to merge but note it only fixes WordPress 5.7 running the Gutenberg plugin (and therefore doesn't need to be backported.)