feat: add "beta" label to libraries v2 content + item bank buttons

[pomegranited]

Description

Adds a "beta" label to the new Libraries V2 components in legacy Studio.

This change affects Course Authors using legacy Studio to add blocks to units.

Supporting information

Relates to: openedx/frontend-app-authoring#1415
Private-ref: FAL-3907

Testing instructions

1. Ensure meilisearch is enabled to enable Libraries V2 content.
2. Disable these waffle flags to use legacy Studio's unit page:
   • contentstore.new_studio_mfe.use_new_course_outline_page
   • contentstore.new_studio_mfe.use_new_unit_page
3. Create/edit a Unit
4. Check that the "beta" label shows under the "Library Content" button.

Deadline

ASAP -- before Sumac cuts

Other information

1. The "Itembank" XBlock is being added by Add "beta" label to Library Content and Problem Bank blockls frontend-app-authoring#1415 ; this will add the "beta" label to this block type too.
2. Designs provided preferred a light green (#E7FEE7) background, but to avoid issues with custom themes, I decided to use built-in colors for background and text.

[openedx-webhooks]

Thanks for the pull request, @pomegranited!

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