Feature Request: Allow Multiple Selections in Dropdown Select Field

### Impacted plugin

Jetpack

### What.

Enhance the existing dropdown field in WordPress.com Forms to support multiple selections, similar to the feature in HTML5.

### How

**Why**
Currently, users can only select a single option from the dropdown field. While checkboxes offer multi-selection, a dropdown with multi-select functionality would improve usability, especially for long lists where checkboxes may take up too much space.

Users expect this feature as it has been available in HTML5 forms since 2014. The absence of this capability forces users to rely on third-party plugins for what is considered a basic form feature.

**Expected Behavior**
A setting to enable multiple selections within the dropdown field.
Users can select multiple options while holding down Ctrl (Windows) or Cmd (Mac).

**User Impact**
Reduces reliance on third-party plugins for basic form functionality.
Enhances form usability, especially for surveys, registrations, and preference selection.
Addresses frustration from users who expect built-in support for a standard feature.

No files, tests, or entry points are named. Start by locating the existing dropdown field in WordPress.com Forms and trace how its settings and rendered HTML are handled; done means users can enable multiple selections and select several options with Ctrl or Cmd.

Written by the indexing model from the issue text.