Dropdown standards for web

[juliafalarini]

Platform
Oppia Web

Describe the request
I would like to request a standard dropdown component for the Design Guide.

Context for the request
I'm currently working on the final mocks for the Practice Session experience project, and this project has dropdowns, but we don't have it on the Design Guide. So I've designed a dropdown for this project, but we should be consistent with this component across the platform. It's most likely that this project will be completed before we have a standard dropdown for it, but I'm filing this issue so this can be updated later, if needed.

DESIGN APPROVAL CHECKLIST -- DO NOT EDIT.

• UX Design approved (Android mocks should include dark mode)
• UX Writing approved
• Design Systems Team approved
• Product approved

For a design project to be completed, it needs approval from UXD, UXW, DS and PM. Please tag the following reviewers when your project is ready (at least one reviewer for each of the 4 bullet points below):

• UXD: N/A (for design style guide projects)
• UXW: @stepmalt
• Design systems: @qluuu / @celestinetales
• PM: @seanlip