This commit renames the following method and function to better represent block theme terminology:
* `WP_Theme::is_block_based()` to `WP_Theme::is_block_theme()`
* `wp_is_block_template_theme()` to `wp_is_block_theme()`
It also changes `wp_is_block_theme()` to be a helper wrapper (sugar syntax) for `wp_get_theme()->is_block_theme();`. Why? To ensure both the method and function behave the same, to help Gutenberg maintain WordPress cross-version compatibility, and to make it less cumbersome to port changes from Gutenberg to Core.
Follow-up to [52069], [52247], [52279].
Props antonvlasenko, costdev, hellofromTonya, noisysocks.
Fixes#54550.
Built from https://develop.svn.wordpress.org/trunk@52330
git-svn-id: http://core.svn.wordpress.org/trunk@51922 1a063a9b-81f0-0310-95a4-ce76da25c4cd