feat: add collection size limit feature

[aannoo]

Summary

This PR adds a collection size limit feature to Decap CMS, allowing administrators to restrict the number of entries that can be created in a folder collection. This is particularly useful for managing content growth and preventing accidental overpopulation of collections.

The implementation:

• Adds an optional limit field to the collection configuration schema
• Introduces a new selectCanCreateNewEntry selector to check if new entries can be created
• Updates the Collection component to respect collection limits
• Includes comprehensive tests for the new functionality

Test plan

The changes have been thoroughly tested:

1. Unit Tests:

npm run test

All tests pass, including:

• Collection component tests
• Selector tests
• Type checking
• Linting

2. Manual Testing:

• Created a collection with a limit of 5 entries
• Verified that the "New Entry" button is disabled after reaching the limit
• Confirmed that non-folder collections are unaffected
• Tested edge cases (undefined limit, null limit)

Checklist

• I have read the contribution guidelines.
• Code is properly formatted and linted
• Tests have been added/updated
• Documentation is updated
• Changes are backward compatible

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