browser-sdk: Add support for render prop for subgrid in video grid

[thyal]

Description

Fixes #1047
Adds a new prop renderSubgridParticipant in the <VideoGrid /> component. This is similar to the renderParticipant render prop that exists already.

The change itself is very small, but I've done several changes to the storybook entries for the video grid to allow for easier testing going forward. Added two entries with fake participants, so that the behaviour of the grid can be tested without joining with a bunch of clients.

Summary:

Related Issue:

Testing

1. pnpm build && pnpm dev
2. Go to the Video Grid Mocked Custom story and check out the custom subgrid tiles

Screenshots/GIFs (if applicable)

Checklist

• My code follows the project's coding standards.
• Prefixed the PR title and commit messages with the service or package name
• I have written unit tests (if applicable).
• I have updated the documentation (if applicable).
• By submitting this pull request, I confirm that my contribution is made
  under the terms of the MIT license.

Additional Information

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🦋 Changeset detected

Latest commit: 85b07f2

The changes in this PR will be included in the next version bump.

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Name	Type
@whereby.com/browser-sdk	Minor

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[sarahrose-ferris]

Looks really good - nice additions with the stories too 👏