Fix: Update Interactivity API Use of Private HTML API Classes to avoid Crash

Previously the HTML API was using a mixture of approaches in its helper classes
for describing spans of text. One was (start, end) and the other was (start, length).
In WordPress/wordpress-develop#5721 these were all normalized to (start, length) but
the Interactivity API was not updated in concert with that change.

In this patch the `inner_html` methods are updated to use the appropriate formats.

A following PR is necessary to update the compat layer so that crashes don't appear
on sites running older versions of WordPress with this updated code.

Author: dmsnell

lib/experimental/interactivity-api/class-wp-directive-processor.php

@@ -195,7 +195,7 @@ public function get_inner_html() {
 		}
 		list( $start_name, $end_name ) = $bookmarks;
 
-		$start = $this->bookmarks[ $start_name ]->end + 1;
+		$start = $this->bookmarks[ $start_name ]->start + $this->bookmarks[ $start_name ]->length + 1;
 		$end   = $this->bookmarks[ $end_name ]->start;
 
 		$this->seek( $start_name ); // Return to original position.
@@ -225,14 +225,14 @@ public function set_inner_html( $new_html ) {
 		}
 		list( $start_name, $end_name ) = $bookmarks;
 
-		$start = $this->bookmarks[ $start_name ]->end + 1;
+		$start = $this->bookmarks[ $start_name ]->start + $this->bookmarks[ $start_name ]->length + 1;
 		$end   = $this->bookmarks[ $end_name ]->start;
 
 		$this->seek( $start_name ); // Return to original position.
 		$this->release_bookmark( $start_name );
 		$this->release_bookmark( $end_name );
 
-		$this->lexical_updates[] = new WP_HTML_Text_Replacement( $start, $end, $new_html );
+		$this->lexical_updates[] = new WP_HTML_Text_Replacement( $start, $end - $start, $new_html );
 		return true;
 	}