Search based on beginning of words

Instead of the search results simply looking to see if a word contains
the term searched for, this makes it so the term has to match the
beginning of one of the words.

For example, let's say the options were:
- First name
- Confirmation time

Previously, a search for "n" would return both, because there is an n in
 the word Confirmation. With this change, only First name would match
 because the word name starts with n.

See the Jira ticket for discussion.

[MAILPOET-3958]

assets/js/src/subscribers/importExport/import/step_data_manipulation/generate_column_selection.jsx

@@ -25,7 +25,8 @@ export default () => {
       // `name` is the label displayed to users
       const matchedChildren = data.children.filter((child) => {
         const label = child.name.toLowerCase();
-        return label.includes(searchTerm);
+        const words = label.split(' ');
+        return words.some((word) => word.startsWith(searchTerm));
       });
       if (matchedChildren.length === 0) {
         // Returning null prevent the entire optgroup from being displayed