Prevent 'BlockTemplatesController' to be initialized in classic themes which don't support templates nor template parts

[Aljullu]

Blocked by #46464.

BlockTemplatesController is a class used to handle block templates and templates parts.

While several of its methods check for the theme to support block templates/block templates parts before running, there are several other methods which don't have those checks, which is easy to cause bugs (see #46429 and #46461).

This issue is about try to only initialize BlockTemplatesController when the theme supports block templates or block template parts. So BlockTemplatesController wouldn't be initialized in classic themes at all.

One challenge is that the check_should_use_blockified_product_grid_templates method needs to be run also in classic themes, so it should be moved out of this class:

woocommerce/plugins/woocommerce/src/Blocks/BlockTemplatesController.php

Lines 217 to 235 in de54195

	/**
	* Checks the old and current themes and determines if the "wc_blocks_use_blockified_product_grid_block_as_template"
	* option need to be updated accordingly.
	*
	* @param string $old_name Old theme name.
	* @param \WP_Theme $old_theme Instance of the old theme.
	* @return void
	*/
	public function check_should_use_blockified_product_grid_templates( $old_name, $old_theme ) {
	if ( ! wc_current_theme_is_fse_theme() ) {
	update_option( Options::WC_BLOCK_USE_BLOCKIFIED_PRODUCT_GRID_BLOCK_AS_TEMPLATE, wc_bool_to_string( false ) );
	return;
	}
	if ( ! $old_theme->is_block_theme() && wc_current_theme_is_fse_theme() ) {
	update_option( Options::WC_BLOCK_USE_BLOCKIFIED_PRODUCT_GRID_BLOCK_AS_TEMPLATE, wc_bool_to_string( true ) );
	return;
	}
	}