feat(dropdowns): make the fluid width an opt-in option

What does this MR do?

We previously made dropdowns' width more flexible by droppping the fixed width in favor of a min and a max width, letting the panel resize itself based on its contents. This restores the fixed width as the default behavior, but makes the fluid width available via the fluid-width prop.

Relevant review app links:

• Default listbox width fixed width: http://2209-fixed-and-fluid-width-dropdowns.34.140.25.101.nip.io/?path=/story/base-new-dropdowns-listbox--default
• Listbox with fluid-width enabled: http://2209-fixed-and-fluid-width-dropdowns.34.140.25.101.nip.io/?path=/story/base-new-dropdowns-listbox--with-long-content

Screenshots

Here's the listbox's with-long-content story with fluid-width disabled and enabled:

Disabled	Enabled

Closes #2209 (closed)