Document category field for browser extension actions

[akirk]

Summary

• Adds category to the friends_browser_extension_actions filter docblock as an optional field
• Actions without a category appear under the default "Actions" header in the extension popup

Test plan

• Verify that a plugin providing an action with a category field has it grouped under that header in the extension popup
• Verify that actions without category still appear under the default "Actions" header
• [ ]

Changelog entry

• Automatically create a changelog entry from the details below.

Add category support for browser extension

Type

• Added - for new features
• Changed - for changes in existing functionality
• Fixed - for any bug fixes
• Removed - for now removed features

Message

Document that a category can be specified.

[github-actions]

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