Horizontal flyout cleanup tasks

[rachel-fenichel]

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Problem

I want to pull the horizontal flyout into a plugin, but it uses some internal APIs.

Request

Find everything that the horizontal flyout uses that is internal and decide whether to make it externally visible, change how the flyout works, or something else. Implement those decisions.

Alternatives considered

No response

Additional context

• targetWorkspace: is @internal on FlyoutBase, needs to become public. Can't just be protected because we also access it from gesture code. Split into Add a getTargetWorkspace method to flyout_base.ts #7461
• isDragTowardWorkspace is internal on FlyoutHorizontal and is called from gesture code. It's already part of the flyout interface. It should not be internal. Split into Remove @internal tag from isDragTowardWorkspace #7462

[gonfunko]

Closing since subtasks have been completed.