Add an experiment for setting the Write tool as default

[richtabor]

Add an experiment within Gutenberg to enable "Write" as the default mode when the tools are available.

The goal here is to experiment with this setting to see if this Write mode helps users more easily update pages that are technically templates, like home and front-page templates, without getting overwhelmed by the default (Design) mode of the editor.

Visual

[richtabor]

Closing this, as it's not as necessary with #65945 as a saved preference.